The present invention provides atazanavir sulfate substantially free of diastereomeric impurities. The present invention also provides atazanavir sulfate having D-tertiary leucine analogues less than 0.1%. The present invention further relates to an improved process for preparing atazanavir sulfate, substantially free of its diastereoisomeric impurities, which comprises of reacting diamino compound (IV) with N-methoxycarbonyl- (L)-tertiary-leucine (V) having D-isomer less than 0.1% to obtain atazanavir base conversion of atazanavir base to atazanavir sulfate by reacting with sulfuric acid and crystallization of atazanavir sulfate from suitable organic solvent(s).
