To crystallize selectively forms I and II of clopidogrel hydrogen sulfate (CHS), nucleation behavior and supersaturation level were studied in mixed solvents such as formic acid/isopropanol, N,N-dimethylformamide/isopropanol, and N-methylpyrrolidone/isopropanol. The effect of supersaturation on induction time and nucleation rate was evaluated by comparison between nucleation mechanisms of seeded and unseeded crystallizations. Experimental results indicate that the polymorph obtained by crystallization depends on the type of seed crystals, amount of seed crystals, and seeding point. Homogeneous nucleation was desirable for the formation of form I. Form II corresponds to primary nucleation, i.e., homogeneous and heterogeneous nucleations.