Effects of different solvents including methanol, ethanol, and n-butylalcohol on the crystallization of avermectin B1a were studied in this paper. In pure crystallization system, solvent effects on nucleation rate and crystal morphology were studied by varying relative supersaturation ratio, which was 1.3-2.8 for methanol, 1.8-2.8 for ethanol, and 2.8-4.5 for n-butylalcohol respectively. Results showed that the nucleation rate of avermectin B1a decreased in the order of crystallization solvents methanol, ethanol, and n-butylalcohol, and the crystal morphology transited from needle like to distorted cube shape. Purification effect of the crystallization process was studied through the segregation coefficient of B2a. The study was carried out by varying the supersaturation ratio as well as the amount of impurity (B2a, 8.7-26.1%, w/w). The segregation coefficient study indicated that the purification effect of the crystallization decreased with increasing supersaturation and purity of the crystallization system. In addition, crystallization with methanol and n-butylalcohol could remove impurities better than with ethanol. (C) 2007 Elsevier B.V. All rights reserved.