A series of 3'-N-acyl-paclitaxel analogues 1a-v were synthesized and their cytotoxicities in vitro against several human tumor cell lines examined. It has been shown that distinct correlation between activity and N-acyl-substituent. The appropriate size of N-acyl group was indispensable for cytotoxicity, and moreover, the presence of beta-substituted conjugated double and triple bond to N-carbonyl generally resulted in increase of cytotoxicities.