Objective To evaluate the performance of electrolyte module in OLYMPUS AU5421 Automatic Biochemical Analy-zer in K+ ,Na+ ,Cl- detection .Methods Electrolyte module in OLYMPUS AU5421 Automatic Biochemical Analyzer was employed to detect K+ ,Na+ ,Cl- ,and their precision ,accuracy ,linearity range ,cross-contamination and the test results of instruments were compared .The linear regression equation and regression coefficients were calculated according to the methods of Clinical and Labo-ratory Standards Institute(CLSI) EP6-A ,EP9-A2 documents .Results The within-run precisions of K + ,Na+ ,Cl- were 0 .328% , 0 .306% ,0 .343% ,respectively ,and their between-run precisions were 0 .780% ,1 .012% ,0 .985% ,respectively .Their regression coefficients of linear equation were all greater than 0 .999 ,and their cross-contamination rate were 1 .066% ,0 .812% and 0 .858% , respectively .Conclusion Electrolyte module in OLYMPUS AU5421 Automatic Biochemical Analyzer modules has advantages of good precision ,accuracy ,linear range ,and low cross-contamination rate .