To evaluate the measurement uncertainty of 23 items of clinical biochemistry. Internal quality control and external quality assessment were used to determine the uncertainty and standard uncertainty for the 23 items detected by Hitachi 7600 automatic biochemical analyzer. The uncertainties met the requirements of CLIA'88 for allowable error before correction except those of Cl-, P, TCH and TG, and correction made those of P, TCH and TG satisfy the requirements. The top-down method contributes to the understanding of the uncertainty of clinical biochemical items, while further efforts have to be performed for the uncertainty assessment standard.