Objective To assay the comparative study and bias estimation of the patient′s serum samples ALT by using the reagents of Beckman Co.and ALT LiquiUV of Human Co.Germany.Methods According to the file of NCCLS-EP9-A of America: every day , take 8 patient samples, analyze each patient samples ALT in duplicate using two kinds of methods separately, and write down the inspection results in 5 days altogether, then evaluate the bias estimation of the different reagents.Results When analyzing the ALT of patient′s fresh serum in the two different reagents, its expected relativity bias could be accepted in the linear range of the method. At the same time, analyzing the different ALT of control mass, its relativity bias could be more than 20%.Conclusion When measuring the serum sample ALT in Beckman′s reagents and Human′s, its relativity bias could be accepted in the linear range.On the other hand, for different control mass have different the relativity bias,the largest bias could reach 27.6%.