Abstract: Influence of injection conditions on matrix effect for GC-MS determination of residual amounts of pesticides in vegetables was studied. The vegetable samples were extracted with acetonitrile,and standard solution was added based on blank matrix. The HP-5MS quartz capillary column was used for GC separation and the selected ion monitoring mode was adopted in MS determination. Chlorpyrifos-methyl was used as internal standard. Matrix enhancement effect could be observed in 29 pesticides. Values of the matrix effect of only 12 pesticides were in the range of 3%-19%. When the inlet temperature and pulse pressure were set at 280 ℃ and 275.8 kPa,respectively,the values of matrix effect of 22 pesticides were reduced to 2%-20%. The optimized injection volume (2 μL) could also reduce the matrix effects of vegetables.