Abstract: As a reactive plasticizer, diallyo(o-)phthalate is also a phthalate ester which should be strictly monitored in food and food related materials. In this paper, a new analytical method using gas chromatographic-mass spectrometric (GC/MS) techniques has been developed for the determination of diallyo(o-)phthalate residue in food plastic packing materials for the first time. Diallyo(o-)phthalate in food plastic packing materials was firstly extracted with n-hexane by ultrasonic methods, and then determined by GC/MS. The detection limit was 10.0 μg/kg, the linear range was from 10~500 μg/L, and the recoveries were between 80% and 110%. The results showed that the method is suitable for the determination of diallyo(o-)phthalate in food plastic packing materials.