In order to evaluate the ecological safety and planting risk of insect resistant transgenic cotton, the composition and diversity of the weed community in transgenic Bt cotton (four Bollgard strains: ’109B’,’154’,’690’ and ’972’) fields were studied by means of the quantitative analysis method in Dafeng of Jiangsu coastal district and Rugao of Jiangsu Changjiang River bank district. There were marked differences in water content of soil due to differences in crop rotation systems (i.e. rotation with rice following dry-cropping) between cotton fields. The experimental design was as follows: five treatments include four Bollgard strains and traditional variety and four replicates in each location. Each plot area is about 60 m2. The traditional variety check was Simian-3 in Dafeng and Sumian-16 in Rugao respectively. All weed species and their number were investigated with the sampled method of five spots per plot and the spots area investigated per plot was 2.5 m2 in Dafeng and 1.25 m2 in Rugao respectively. The average of weed species and number per square meter for each treatment was calculated. The parameters and components analysis of weed community for each treatment and coefficients of weed species/community similarity between each of four Bollgard strains and traditional variety check were calculated and analyzed also. The results showed: 1) no significant difference in weed species was found between each of four transgenic Bollgard cotton strains and the traditional variety checked in the same location. The dominant species and their abundance were very similar among them.2) the parameters of weed community structure included species richness(S)&amp;#65292;total number of individuals(N), community diversity (H&amp;#8242;), evenness(J), and concentration(C) in each of the transgenic Bt cotton (four Bollgard strains) fluctuated similarly to the ones in the traditional variety in the same date and same location. The results of comprehensive analysis for two locations showed that the value of community diversity(H&amp;#8242;) and evenness(J) in transgenic Bt cotton was a little higher than the ones in traditional cotton check, but the value of concentration(C) in transgenic Bt cotton was a little lower than the ones in traditional variety check. 3) the path analysis of diversity index(H&amp;#8242;) with other parameters showed that it all was dominant concentration(C) and evenness(J) that mainly decided the change of diversity index(H&amp;#8242;) for weed community in five treatments. Diversity index(H&amp;#8242;) would be reduced with increasing of dominant concentration(C) and decreasing of evenness(J) for all treatments.4) the coefficient of weed species similarity(V1) and the coefficient of weed community similarity(V2) between each of four transgenic Bt Bollgard cotton strains and traditional variety check was determined by the Sorensen’s formula and all very high. The value of V 1 is 0.718 0&amp;#65293;0.844 5 in Dafeng and 0.489 4-0.603 7 in Rugao&amp;#65292;and V2 is 0.451 1-0.598 1 in Dafeng and 0.329 3-0.484 7 in Rugao respectively.  From the above results, it is clear that the stability of weed community of transgenic Bt cotton field (four Bollgard cotton strains) is not different significantly than traditional cotton check and its weed risk increased by planting transgenic Bt cotton is not obvious. There was a close correlation among the structure of weed community, crop rotation system and geographical factors. The main factor that affected the weed occurrence and distribution pattern of weed infestation and community in the cotton fields was the difference of soil water content, which resulted from the different cropping system.