The nucleation effects of branched poly(lactic acid), having the castor oil core, on the crystallization behavior of the bacterial polyester, poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), were investigated in this study by using differential scanning calorimetry (DSC) and polarized optical microscopy (POM). Enhanced crystallization of PHBV was observed in the presence of the branched poly(lactic acid) than in linear poly(lactic acid). Crystallization of the PHBV/branched poly(lactic acid) blend from the melt was completed during the cooling process at 10 degrees C/min. The crystallization half time (t(1/2)) of the PHBV/branched poly(lactic acid) blend decreased significantly. By the addition of the branched poly(lactic acid), the spherulite size of PHBV became much smaller than that of neat PHBV.