Abstract: It was reported that in the determination of dimethyl fumarate (DMFU) by GC-MS, the use of dimethyl malonate (DMM), diethyl malonate (DEM) and dimethyl glutarate (DMG) as internal standards was studied and discussed. HP-5MS capillary column with programmed temperature elevation (50 ℃-300 ℃) was used in GC separation, and whole scanning mode (in the range of 50-400 amu) as well as selected ions monitoring mode were used in MS detection. It was found that ① clear separation between DMFU and the 3 internal standards was obtained, ② values of peak area of the DMFU were keeping in persistent stability ③ difference of retention times between DMFU and the 3 internal standards were less than 2.8 min. Linearity range of DMFU found were in the range of 0.005-5.0 mg·L-1. The proposed method was applied to the determination of DMFU in substantial samples, the results obtained by using either one of the 3 internal standards showed no significant difference. Therefore, either of the 3 internal standards could be used for routine determination of DMFU in various daily-use products.