Abstract: Pot experiments were conducted to determine the response of Amaranthus retrofluxes, Digitaria sangunalis, Echinochloa crus-galli, Eleusine indica, Abutilon theophrasti to nicosulfuron. A. retrofluxes and E. crus-galli were more susceptible than other species, while A. theophrasti was the most tolerant species. Addition of Silwet L-77 increased the efficacy of nicosulfuron on all weed species except D. sangunalis. The influence of leaf stage on the performance of nicosulfuron was examined on D. sangunalis. Results showed that application to D. sangunalis at 3 to 5-leaf stage achieved 82% control even under the application rate 10 g ai/hm2. Control efficacy was significantly decreased with increasing growth stage from 5-leaf to 7-leaf. The rainfastness of nicosulfuron was examined on A. retrofluxes and D. sangunalis. A rainfall of 3 mm was applied 1.5, 3.0 and 6.0 h after spray using a rain simulator. The activity of nicosulfuron was reduced significantly by rainfall 1.5 and 3.0 h after spraying, but not reduced 6.0 h after application. Addition of Silwet L-77 improved the rainfastness of nicosulfuron on A. retrofluxes, not on D. sangunalis.