To study the toxic effects of ammonia on the aquatic specials, four ammonia levels (0.071 mg·L-1, 0.143 mg·L-1, 0.284 mg·L-1, and 0.427 mg·L-1) with one control group were set for Cyprinus carpio for research. The samples were tested at the 1st, 7th, 14th, 28th and 35th day. SOD、GSHPx activities and MDA content of the liver, pancreas, and kidney were determined. The results showed that with an increase of the ammonia concentration, the activities of SOD and GSH-Px in the pancreas first increased and then decreased, and the MDA content exhibited a rising trend. It showed very significantly that ammonia exposure affected the content of MDA and activity of SOD by variance analysis (P<0.01),and they had a dose-effect relationship. Significantly, ammonia exposure affected the activities of SOD and GSH-Px in the kindey by variance analysis (P<0.05),and it was very remarkable that ammonia exposure affected the content of MDA in the kidney (P<0.01), and they had a dose-effect relationship. With the extension of ammonia exposure time, the activities of SOD in the liver, pancreas, and kidney, and GSH-Px in the kidney were promoted then suppressed at a low concentration. However, these activities were suppressed at a high concentration. It had no significant difference by variance analysis(P>0.05). Ammonia could reduce the oxidation resistance of the liverpancreas, and kidney in the test concentration range, and the content of MDA and activity of SOD could be used as an effective biomarker for toxicity assessment of ammonia to Yellow River Cyprinus carpio.