Bioassay-directed fractionation of the chloroform extract of the neem oil has resulted in the isolation of one new esters derivatives named 2-Octadecanoic acid-4-Palmitic acid-2,4-Pentanediyl ester (compound 1). Its structure is deduced on the basis of spectroscopic evidence. The test of in vitro spermicidal activity of the compound 1 shows that the compound 1 can kill all sperms of Kunming mice in vitro in less than 20 sec at the concentration of 2mg/ml.