Abstract: In this paper, the synthesis and analgesic activities of some derivatives of 4-methoxycarbonyl fentanyl are reported, in which the 4-N-phenyl group or both the 4-N-phenyl group and the 1-β-phenyl group were replaced by some nonaromatic groups. In the last case, some compounds showed very strong analgesic activity. For example, the analgesic activities of compounds 4 and 6 were shown to be 695 and 818 times more potent than that of morphine respectively. The structure-analgesic activity relationships were discussed.