Abstract: The difference and mechanism of tolerance of various wheat varieties to florasulam were measured by dish dipping and enzyme assay in laboratory.The results demonstrated that the roots of various wheat varieties showed different tolerance to florasulam, and Ji’ning 13 was the most sensitive variety (IC50 value 0.221 2 μg/mL).The tolerance of Yannong 15,Zao 20-28 and Ji’nan 17 were similar to that of Ji’ning 13 (with the tolerance ratio 1.09-1.99),while Taishan 9818,Shannong 6 and Weimai 8 showed relatively high tolerance (with the tolerance ratio 10.33-13.92).Weimai 8 and Ji’nan 17 were chosen for enzyme assay.The results showed that the sensitivity of Weimai 8 and Ji’nan 17’s ALS in vitro to florasulam was not significantly different, even though the difference in tolerance between the two varieties was significant.The in vivo ALS activity of the two varieties could recover rapidly,and the recovery of Weimai 8 was faster than that of Ji’nan 17.The GSTs activity of Weimai 8 showed a rapider and stronger response than that of Ji’nan 17 after being treated by florasulam.The relative GSTs activity of Weimai 8 reached the peak value 1.460 in the 2nd day but that of Ji’nan 17 reached the peak value 1.398 in the 3rd day.The results of present study suggested that the mechanism of various wheat varieties showed different tolerance to florasulam may not related with the sensitivity of ALS,but with the enhancement of wheat’s GSTs activity.