Abstract: In order to study the mechanism of different tolerance of two wheat (Triticom aestivum L.) cultivars against mesosulfuon, the trial was carried out by the comparison between thesensitivities of acetolactate synthase (ALS) to mesosulfuon. The difference sensitivity between two wheat cultivars to mesosulfuon was observed in greenhouse experiments. Seedling height was reduced by folia application of mesosulfuron plus mefenpyr-diethyl. The level of ALS enzyme extracted from shoot in R-6 was higher than that in S-18. The specific activity of ALS in R-6 was increased to ratio 1.29 and 1.58 by mefenpyr-diethyl treatment in the concentration of 200 and 400 μmol/L, while the ratio in S-18 was 0.25 and 0.15. From these experiments, it was concluded that R-6 was more tolerant than S-18 to mesosulfuron, and the difference level and specific activity of ALS induced by mefenpyr-diethyl may partly contribute to the differential tolerance of the two wheat cultivars to mesosulfuron.