Abstract: IC was applied to the determination of sulfur trioxide in gypsum. The sample of gypsum was dissolved with water by ultrasonic extraction,and then filtered with microporous membrane. IonPac AS11-HC anion exchange column (4 mm×250 mm) was used for separation with gradient elution of free agents,and determined with conductivity detector. Linear relationship between values of peak area and mass concentration of sulfate was kept in the range of 20.0-500.0 mg·L-1. The proposed method was applied to the analysis of standard samples of gypsum (GBW 03109,GBW 03111),giving results in consistency with the certified values. Values of recovery found by standard addition method were in the range of 80.6%-109%,and values of RSD′s (n=6) in the range of 1.3%-6.8%.