Abstract: A bioassay method of Maiguning, a new sulfonylurea herbicide, on the maize main root length was reported, using the wide planted tolerant maize Zea mays L. (CAU 108) as indicative plant.This proceduce includes: germination, dissemination, covering 1cm sand on the seeds, herbicide application, covering another 1cm sand and cultivating 3 days in the darkness at 25 ℃. The study showed that there's a good linear relationship between the logarithm of concentration of herbicide Maiguning and the probability corresponding to the inhibition percentage of the main root length. The sensitivity of 16 varieties of widely planted maize to Maiguning was studied by bioassay methods The results indicated that Yedan No. 12 is sensitive to Maiguning, while CAU 3138 is the most tolerant. IC50 values of Maiguning to all these maize species are higher than 7.38 μg·kg -1 The sensitivities of different variety to Maiguning are lower than chlorsulfuron and higher than nicosulfuron