Abstract: On the base of the mass spectrograms of 4 sym-triazine herbicides (i.e., terbuthylazine, sebuthylazine, trietazine and propazine) obtained under the condition of EI ionization, a comparative study on the characteristic ions formed in the fragmentation processes, the courses of fragmentation and their mechanism of the 4 compounds, was made. It was found that in spite of having the same triazine mother structure and similar substituents, slight structural differences were existed among the 4 compounds, leading to significant differences in their mass spectrograms. These phenomena could be used as basis for differentiation and certification of derivatives or isomers of compounds of same category.