Abstract: Objective To evaluate the capacity of laboratories participated in the proficiency testing (PT) of determination potassium in serum and improve the quality of testing, and put forward technical suggestions for unsatisfied laboratories. Methods According to the requirements of CNAS related documents, the homogeneity and stability of the real PT sample were evaluated by one-way ANOVA and t test, respectively. The values of real PT samples were assigned by reference method which was used in PT results assay. It is required that the deviation of value of real PT samples (code:2, 3, 5) between the measured value and the assigned value shall be within ±15.0%. The precision of values for all samples should not be greater than 3.0%. Results All the laboratories submitted valid data according to the requirements. Only one laboratory did not meet the requirements, and the satisfaction rate was 90.9%. Conclusion The ability of most of laboratories are accurate and reliable.