A pneumatic tire comprising: an annular tread portion extending in a tire circumferential direction; a pair of sidewall portions disposed on both sides of the tread portion; and a pair of bead portions disposed inward of the sidewall portions in the tire radial direction, and comprising a carcass layer mounted between the pair of bead portions, the carcass layer comprising, as carcass cords, a plurality of composite cords that are made of a polyethylene terephthalate fiber yarn having a fineness of from 420 dtex to 1200 dtex and an aromatic polyamide fiber yarn having a fineness of from 520 dtex to 1200 dtex, characterized in that the plurality of composite cords have a total fineness of from 940 dtex to 2400 dtex, and a twist coefficient K expressed by Equation that falls within a range of from 1600 to 2400, the composite cords having an intermediate elongation within a range of from 2.0% to 4.5% with a load of 2.0 cN, having a dimensional stability index of from 2.5% to 6.0% which is a sum of a dry heat shrinkage rate at 150°C and the intermediate elongation with the load of 2.0 cN, and having a ratio of an initial elastic modulus M1 at 100°C within a load range of from 0 N to 44 N with respect to an initial elastic modulus M0 at 25°C within a load range of from 0 N to 44 N, the ratio being within a range of from 76% to 94%; K = T √ D where T: cable twist count (twists/10 cm), and D: cord total fineness (dtex); wherein the intermediate elongation with a load of 2.0 cN is the elongation ratio (%) of a sample cord measured where a tensile test is performed, compliant with the "Testing Methods for Chemical Fiber Tire Cords" of JIS-L1017, under conditions of a grip spacing of 250 mm and a test speed of 300 ± 20 mm/minutes with an application of load of 2.0 cN/dtex; wherein the dimensional stability index is the sum of the intermediate elongation with a load of 2.0 cN and the dry heat shrinkage rate at 150°C, where the dimensional stability is higher when the sum is smaller; wherein the dry heat shrinkage rate at 150°C is the dry heat shrinkage rate (%) of the sample cord measured under conditions that, compliant with the "Testing Methods for Chemical Fiber Tire Cords" of JIS-L1017, a sample length is set at 500 mm and heating condition is set at 150°C for 30 minutes; and wherein the initial elastic modulus M0 or M1 is the initial elastic modulus (cN/dtex) in a load range of from 0 N to 44 N where a tensile test is performed, compliant with the "Testing Methods for Chemical Fiber Tire Cords" of JIS-L1017, under the conditions of a grip spacing of 250 mm and a test speed of 300 ± 20 mm/minutes, where a reduction in elastic modulus due to tire temperature rising is smaller where the ratio of the initial elastic modulus M1 to the initial elastic modulus M0 is higher. an annular tread portion extending in a tire circumferential direction; a pair of sidewall portions disposed on both sides of the tread portion; and a pair of bead portions disposed inward of the sidewall portions in the tire radial direction, and comprising a carcass layer mounted between the pair of bead portions, the carcass layer comprising, as carcass cords, a plurality of composite cords that are made of a polyethylene terephthalate fiber yarn having a fineness of from 420 dtex to 1200 dtex and an aromatic polyamide fiber yarn having a fineness of from 520 dtex to 1200 dtex, characterized in that the plurality of composite cords have a total fineness of from 940 dtex to 2400 dtex, and a twist coefficient K expressed by Equation that falls within a range of from 1600 to 2400, the composite cords having an intermediate elongation within a range of from 2.0% to 4.5% with a load of 2.0 cN, having a dimensional stability index of from 2.5% to 6.0% which is a sum of a dry heat shrinkage rate at 150°C and the intermediate elongation with the load of 2.0 cN, and having a ratio of an initial elastic modulus M1 at 100°C within a load range of from 0 N to 44 N with respect to an initial elastic modulus M0 at 25°C within a load range of from 0 N to 44 N, the ratio being within a range of from 76% to 94%; K = T √ D where T: cable twist count (twists/10 cm), and D: cord total fineness (dtex); wherein the intermediate elongation with a load of 2.0 cN is the elongation ratio (%) of a sample cord measured where a tensile test is performed, compliant with the "Testing Methods for Chemical Fiber Tire Cords" of JIS-L1017, under conditions of a grip spacing of 250 mm and a test speed of 300 ± 20 mm/minutes with an application of load of 2.0 cN/dtex; wherein the dimensional stability index is the sum of the intermediate elongation with a load of 2.0 cN and the dry heat shrinkage rate at 150°C, where the dimensional stability is higher when the sum is smaller; wherein the dry heat shrinkage rate at 150°C is the dry heat shrinkage rate (%) of the sample cord measured under conditions that, compliant with the "Testing Methods for Chemical Fiber Tire Cords" of JIS-L1017, a sample length is set at 500 mm and heating condition is set at 150°C for 30 minutes; and wherein the initial elastic modulus M0 or M1 is the initial elastic modulus (cN/dtex) in a load range of from 0 N to 44 N where a tensile test is performed, compliant with the "Testing Methods for Chemical Fiber Tire Cords" of JIS-L1017, under the conditions of a grip spacing of 250 mm and a test speed of 300 ± 20 mm/minutes, where a reduction in elastic modulus due to tire temperature rising is smaller where the ratio of the initial elastic modulus M1 to the initial elastic modulus M0 is higher. T: cable twist count (twists/10 cm), and D: cord total fineness (dtex);