Abstract: A sensitive liquid chromatography- tandem mass spectrometry method( LC- MS / MS) was developed for the determination of Nonylphenol( NP) in polyunsaturated fatty acids microencapsulated powder.The Nonylphenol in samples was extracted with the mixture of cyclohexane and ethyl acetate( 1 ∶ 1,v / v).The extractions were purified by centrifuge with low temperature and LC- MS / MS analysis was performed by positive ion electrospray ionization applying multiple reaction monitoring with internal standard method for quantitative analysis.The condition of the sample extraction was optimized by the orthogonal experiment.The optimized fermentation conditions were found to be: extraction 1 time,ultrasonic time 15 min,1.5 g samples and vortex time 20 s.The limits of quantitation( LOQ) was 0.3 μg /kg,and the method showed good linearity between 1~150 ng /m L( r = 0.9998).The results of the samples were 36.21~123.98 μg / kg.The recoveries were between 94.6% to 108.1% and RSD were between 3.9% to14.3%.The method was simple,quick and accurate.The method was proved to be appliable in the determination of NP in polyunsaturated fatty acids microencapsulated powder.