Abstract: The study aims to make clear the composition and eco-niches of the major weeds during the early growth period of late rice, and to predict the evolution of weed communities under the action of herbicides. Weed communities in late rice fields of Ningbo were studied by using diagonal five-point sampling method. The results showed that Echinochloa crusgalli, Leptochloa chinensis, Ammannia arenaria, Cyperus difformis, Sagittaria pygmaea, Leersiahexandra swartz were the main populations in this area. The niche breadth and niche overlap of the species were calculated. The results indicated that E. crusgalli and L. chinensis had time and horizontal niches breadth >0.90, their niches overlaps were the highest (0.9088, 0.8261), which indicated that they were the dominant species in the early growth stage of late rice. A. arenaria had minimum time niche breadth (0.4955), but had higher horizontal eco-niches overlaps together with E. crusgalli and L. chinensis, which were 0.7201 and 0.6990 respectively. This indicated that A. arenaria had a later germination, but with strong competitive advantage. Neglecting of prevention on A. Arenaria will lead it to be the dominant species in the late stage of rice growth.