An ammonia-oxidizing strain of Penicillium sp., M25-22 which was isolated from chicken feces compost, was studied to make clear its characteristics of ammonium oxidation and provide References for its application in aerobic composting. In media containing (NH4)2SO4 at a concentration of 0.5 g 100 mL-1, mycelium weight and utilization ratio of ammonium nitrogen increased markedly during the 1 st 5 d, concentration of nitrate increased markedly from the 3rd to the 5th d and concentration of nitrite remained at a low level. The strain showed the ability to oxidize ammonium nitrogen growing in media containing glucose, sucrose, starch, or cellulose as the sole carbon source and to oxidize the negative trivalent nitrogen of ammonium sulfate, peptone, acetamide, urea, or L-aspartate. Slowly available carbon or nitrogen sources such as starch, cellulose, and peptone were beneficial to ammonium oxidation. In media containing sucrose as the sole carbon source and ammonium sulfate as the sole nitrogen source, nitrate formation was the most pronounced with sucrose present at a concentration of 12 g L-1 and ammonium nitrogen present at a concentration of 2.438 mg mL-1, at the initial pH level of 7.5 and at 30℃.