A cathode active material comprising lithium transition metal phosphate particles represented by Chemical Formula 1, wherein the lithium transition metal phosphate particles comprise: a first secondary particle formed by agglomeration of two or more first primary particles; and a second secondary particle formed by agglomeration of two or more second primary particles in the first secondary particle, wherein the average particle diameter of the first primary particle is greater than the average particle diameter of the second primary particle, and the average particle diameter of the particle is measured by using a laser diffraction method, wherein the average particle diameter of the first primary particle is in a range of 200 nm to 500 nm, wherein the average particle diameter of the second primary particle is less than 50 nm, wherein the average particle diameter of the first secondary particle is in a range of 15 μm to 30 μm, and wherein the first secondary particle is porous, Chemical Formula 1 Li 1+a M1 1-x M2 x (PO 4.b)X b where M1 represents at least one element selected from the group consisting of iron (Fe), manganese (Mn), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), and magnesium (Mg); M2 represents at least one element selected from Groups 2 to 15 elements excluding M1; X represents at least one element selected from the group consisting of fluorine, sulfur, and nitrogen; and -0.5≤a≤+0.5, 0≤x≤0.5, and 0≤b≤0.1, wherein the internal porosity of the cathode active material is in a range of 30% to 40%, and wherein the internal porosity is defined below: Internal porosity = volume of pores per unit mass / specific volume + volume of pores per unit mass, wherein the average particle diameter of the particles is the particle diameter at 50% in a cumulative particle diameter distribution, wherein the internal porosity is measured by nitrogen absorption BET. a first secondary particle formed by agglomeration of two or more first primary particles; and a second secondary particle formed by agglomeration of two or more second primary particles in the first secondary particle, wherein the average particle diameter of the first primary particle is greater than the average particle diameter of the second primary particle, and the average particle diameter of the particle is measured by using a laser diffraction method, wherein the average particle diameter of the first primary particle is in a range of 200 nm to 500 nm, wherein the average particle diameter of the second primary particle is less than 50 nm, wherein the average particle diameter of the first secondary particle is in a range of 15 μm to 30 μm, and wherein the first secondary particle is porous, Chemical Formula 1 Li 1+a M1 1-x M2 x (PO 4.b)X b where M1 represents at least one element selected from the group consisting of iron (Fe), manganese (Mn), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), and magnesium (Mg); M2 represents at least one element selected from Groups 2 to 15 elements excluding M1; X represents at least one element selected from the group consisting of fluorine, sulfur, and nitrogen; and -0.5≤a≤+0.5, 0≤x≤0.5, and 0≤b≤0.1, wherein the internal porosity of the cathode active material is in a range of 30% to 40%, and wherein the internal porosity is defined below: Internal porosity = volume of pores per unit mass / specific volume + volume of pores per unit mass, wherein the average particle diameter of the particles is the particle diameter at 50% in a cumulative particle diameter distribution, wherein the internal porosity is measured by nitrogen absorption BET.