The experiment was carried out to determine the direct antagonistic ability (donor - receiver bioassay) of OM rice varieties (4498, 3536, 5930, 2395, 6976, 7347, 5451, and N406) on the growth of barnyardgrass, red sprangletop, grasslike fimbry, mustard green, and watercress in laboratory conditions. Ten germinated rice seeds were placed on petri dishes for 48 hours. After that, 10 just-germinated seeds of barnyardgrass, red sprangletop, grasslike fimbry, mustard green, and watercress were put into intercropping with the rice seedlings in the breeding chamber (25°C, 80-100 μE m^-2 s^-1) for 12 hours of photosynthesis in the next 48 hours. The inhibition percentage on the shoot and root length of the tested plants when grown together with different rice varieties were recorded. The results showed that OM rice varieties (4498, 3536, 5930, 2395, 6976, 7347, 5451, and N406) addressed the inhibitory effect on the growth of tested plants at different levels. In particular, OM 5930 caused the inhibition at the highest average (47.0%) on the shoot and root length of the tested plants compared with the remaining rice varieties, Specifically, barnyardgrass (28.9 and 40.4%), red sprangletop (47.1 and 48.7%), grasslike fimbry (49.8 and 57.5%), mustard green (45.0 and 46.6%), and watercress (44.8 and 58.3%), respectively. The results exhibited that all tested rice varieties might contain allelochemicals, which released from the roots of rice and affected the growth and development of the tested germinated seeds, including invasive weeds as barnyardgrass, red sprangletop, and grasslike fimbry. The application of these rice varieties in the weed biological control program via allelopathy in rice is very useful and promising in the future.