A process for the preparation of carfilzomib of Formula I or a pharmaceutically acceptable salt thereof: comprising: a) reacting a compound of Formula II with a compound of Formula III or a salt thereof in presence of a coupling agent (C1), an additive (A1) and a base (B1) in a suitable solvent (S1) to obtain a compound of Formula IV, b) reacting the compound of Formula IV with a suitable acid to obtain a compound of Formula V or a salt thereof, c) reacting the compound of Formula V or a salt thereof with a compound of Formula VI in presence of a coupling agent (C2), an additive (A2) and a base (B2) in a suitable solvent (S2) to obtain a compound of Formula VII, d) deprotecting the compound of Formula VII in presence of a suitable deprotecting agent to obtain a compound of Formula VIII, e) reacting the compound of Formula VIII with a compound of Formula IX or a salt thereof in presence of a coupling agent (C3), an additive (A3) and a base (B3) in a suitable solvent (S3) to obtain a compound of Formula X, f) reacting the compound of Formula X with a suitable acid to obtain a compound of Formula XI or a salt thereof, and g) reacting the compound of Formula XI or a salt thereof with a compound of Formula XII or a salt thereof in presence of a coupling agent (C4), an additive (A4) and a base (B4) in a suitable solvent (S4) to obtain carfilzomib or a pharmaceutically acceptable salt thereof, wherein the coupling agent (C1), (C2), (C3) and (C4) are used in an amount of about 0.9 to about 1.3w/w mole equivalents per mole of Formula III, Formula IV, Formula VIII and Formula XI respectively; wherein the additive (A1), (A2), (A3) and (A4) are used in an amount of about 0.05 to about 0.5w/w mole equivalents per mole of Formula III, Formula IV, Formula VIII and Formula XI respectively. a) reacting a compound of Formula II with a compound of Formula III or a salt thereof in presence of a coupling agent (C1), an additive (A1) and a base (B1) in a suitable solvent (S1) to obtain a compound of Formula IV, b) reacting the compound of Formula IV with a suitable acid to obtain a compound of Formula V or a salt thereof, c) reacting the compound of Formula V or a salt thereof with a compound of Formula VI in presence of a coupling agent (C2), an additive (A2) and a base (B2) in a suitable solvent (S2) to obtain a compound of Formula VII, d) deprotecting the compound of Formula VII in presence of a suitable deprotecting agent to obtain a compound of Formula VIII, e) reacting the compound of Formula VIII with a compound of Formula IX or a salt thereof in presence of a coupling agent (C3), an additive (A3) and a base (B3) in a suitable solvent (S3) to obtain a compound of Formula X, f) reacting the compound of Formula X with a suitable acid to obtain a compound of Formula XI or a salt thereof, and g) reacting the compound of Formula XI or a salt thereof with a compound of Formula XII or a salt thereof in presence of a coupling agent (C4), an additive (A4) and a base (B4) in a suitable solvent (S4) to obtain carfilzomib or a pharmaceutically acceptable salt thereof, wherein the coupling agent (C1), (C2), (C3) and (C4) are used in an amount of about 0.9 to about 1.3w/w mole equivalents per mole of Formula III, Formula IV, Formula VIII and Formula XI respectively; wherein the additive (A1), (A2), (A3) and (A4) are used in an amount of about 0.05 to about 0.5w/w mole equivalents per mole of Formula III, Formula IV, Formula VIII and Formula XI respectively. <EOS>