Abstract: alkylsulphur-5-cyano-6-amido-pyrimidine derivatives for herbicide was reported by electron impact mass spectrometry(EIMS). EI mass spectra showed that the molecular ion peaks of all above compounds were found because of long conjugated structure. The fragmentation mechanisms of above compounds were proposed on the base of the abundance of fragment ions. It showed that the fragment center is on the 2-substituted-alkylsulphur which is consisted of the reactive center.