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, an additive and a base in a suitable solvent 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, an additive and a base in a suitable solvent 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, an additive and a base in a suitable solvent 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, an additive and a base in a suitable solvent to obtain carfilzomib or a pharmaceutically acceptable salt thereof, wherein the coupling agent,, and 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,, and 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, an additive and a base in a suitable solvent 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, an additive and a base in a suitable solvent 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, an additive and a base in a suitable solvent 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, an additive and a base in a suitable solvent to obtain carfilzomib or a pharmaceutically acceptable salt thereof, wherein the coupling agent,, and 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,, and 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.