BiologicalAmmonia removal (nitrification), the oxidation of ammonia to nitrate catalyzed by bacteria, is a key part of global nitrogen cycling. In the first step of nitrification, chemolithoautotrophic ammonia oxidizer transform ammonia to nitrite, which subsequently oxidized to nitrate by nitrite oxidizing bacteria. This process can be affected by several factors. In this study the effect of influent CODon biological ammonia removal in a benchscale biological reactor was investigated. Experiments were carried out using synthetic wastewater. The initial ammonium concentration was 25mgNH+ 4-N L-1. The effect ofCODbetween 247.55+1.8 and 601.08+3.24 mgL-1on biological ammonia removal were investigated by varying the COD loading supplied to reactor. Fromthe results obtained in this study it could be concluded in the range of 247.55+1.8 to 351.35+2.05mgL-1, there is a direct relationship between amount of COD and ammonia removal. Howevermore than 351.35+2.05 up to 601.08+3.24mgL-1was found an indirect relationship between them.