Abstract:In order to identify the effectiveness of different herbicides on the shift of weed community, herbicides such as the mixture of acetochlor and bensulfuron-methyl, the mixture of propisochlor and bensulfuron-methyl and the mixture of mefenacet and bensulfuron-methyl etc were applied sequently at recommended dosages for four years to control weeds in transplanted rice. The biomass of weeds was investigated in 50 days after transplanted rice once a year and the population of weeds in response to herbicide application was observed. The result showed that there was a close relationship between the structure of weed community and the number of year of herbicide applying. The linear or curse relationship between the abundance of target weed and the years of the herbicide applying, and trend was best fit by the model, Y=a+bx or Y=ax2+bx+c. After four years continuous use of herbicides, the structure of the weed community seemed to difference in different treatments. The marked difference of weed homogeneous degree and index of biological diversity occurred between the combined herbicides and single herbicide. The homogeneous degree and index of biological diversity of weed communities with the combined herbicides were 0.40-0.43 and 1.57-1.84 respectively, and were less than the other treatments in paddy fields. Their weed communities was consist of Lindernia procumbens, Ammannia baccifera, Leptochloa chinensis and Juncellus serotinus and the fresh weight of these weeds were significantly lower than that of other treatments.