Abstract: The study was conducted to identify critical nitrogen percent in grain and optimum nitrogen and water level combination in low land rice in central Bihar where farmers apply meager amounts of fertilizer nitrogen. The critical limit technique and regression equation were used. There were significant relation ships between yield, grain nitrogen, fertilizer nitrogen and water levels (r= 0.942, 0.947). The critical level of grain nitrogen content found was 0.92% and was falling near to the optimum combination. The optimum combination found was 80 kg N ha-1 and 8 cm water level in relation to a yield of 5500 kg ha-1. This GN index is useful for optimizing the available water resources as well as external nitrogen requirement of the rice crop and the technique used for derivations were practically simple.