Abstract: A simple, reliable and rapid method was built for determining of neptunium in 30% tributyl phosphate/kerosene and 0.5 mol/L thenoyltrifluoroacetone in xylene by inductively coupled plasma mass spectrometry(ICP-MS) equipped with a sample for organic phase. The instrumental and operating parameters were optimized with the standard solution of organic matrices. The effect of matrix on determination of neptunium was discussed in detail by ICP-MS. The internal standard element of Th was employed to eliminate the signal effect induced by matrix and instrument signal drift. The results suggest that the detection limit(3s, n=10) of the method is no greater than 1×10^-8 g/L, the relative standard derivations(sr) is 0.8%(n=10), and the recoveries vary from 95.5%-115%. This method is applicable for the determination of neptunium in organic phase from the commercial spent fuel reprocessing.