A study was carried out to determine the effect of ammonium thiosulfate (ATS), a byproduct of petroleum refinery, on nitrification and to find its possible mechanism. These objectives were addressed by applying nitrogen fertilizer (urea) in combination with application of thiosulfate at different rates and simultaneously measuring changes in urea-N, NH₄⁺,N0₃^- and NO₂^- at various time in the course of incubation as well as thiosulfate transformation. The results indicated that ATS applied to the soil was rapidly hydrolyzed with an accumulation of tetrathionate and sulfate. Urea was completely hydrolyzed within 3 days with a rapid increase of ammonium content in the soil. The formation of nitrate from ammonium was markedly suppressed by the application of ATS. ATS is a promising nitrifieation inhibitor. At the highest rate of ATS (2500μg ATS/g soil) nitrification inhibition over 14 days was 82%. The inhibition of ATS in this study occurred in the initial period of nitrification and was caused by thiosulfate and its oxidation products such as tetrathionate.