An improved enzymatic end-point method for determination of plasma ammonia was reported. Semi-micromethod were used with a total incubation volume 1 165 μl. Mix 125 μl EDTA Na₂-anticoagulation plasma and 1 000 μl PBS-substrate-solution containing 1.9 kU/L LDH (pH 8.0), then add 40 μl GLDH (≥12×10⁴ U/L) to start the reaction. Incubation periods were shortened to 5 min. The limited detection for ammonia in plasma is 4 μmol/L, and this method is linear from 6 to 200 μmol/L. The method is accurate and reliable. The analytical recovery is from 90% to 105%. The within-run CV% was≤5%. The routine condition variance of the method ranged from 5% to 10%. It was found that 1.9 kU/L LDH was sufficient to eliminate the decreace of NADH by endogenouse pyruvate and LDH in plasma. The normal fasting ammonia in plasma is 10～70 μmol/L.