Abstract: Although crop yields can be improved by engineered genetic modification of plants, ecological concerns remain about persistence of such crops in the wild in the event of dispersal from cultivated habitats with unintended effects, like adverse impacts on weed community composition and diversity in the fields. This study aimed to assess the effects of GMC (genetically modified crop) rice with CryIAb gene on the diversity, structure and composition of weed communities in rice fields and using conventional indica rice ("MH86") as the control. A total of 16 plots in semi-wild cultivation and conventional cultivation conditions were analyzed. The conventional cultivation treatment was in accordance with local farming practice, while in the semi-wild cultivation was non-farming practice with irrigation. All weed species and numbers collected from five sample spots per plot were investigated. An investigated sample spot was 0.25 m2 in the semi-wild plots and 1.0 m² in the conventional cultivation plot. The results showed: no significant difference in weed species between transgenic CryIAb ("Mfb") rice and non-transgenic ("MH86") rice fields under the same cultivation pattern. While high weed frequency and density were observed in semi-wild plots compared to conventional cultivation plots, no differences in frequency and density were noted between two varieties. Statistical analysis suggested that species richness and Shannon-Wiener diversity index were greater in semi-wild plots than in conventional cultivation plots. All the applied multivariate methods of analysis revealed that weed community composition was significantly different between the two cultivation conditions. There was an unobvious difference between transgenic and non-transgenic rice communities in terms of the indices of weed species richness, evenness, dominance and diversity. The results suggested that the diversity and stability of weed community in transgenic rice fields did not significantly differ from those in traditional rice fields.