A method was developed for the determination of trace free copper and zinc in serum sample by GFAAS after two-step precipitation. The serum and ethanol were mixed with volume ratio of 1:2. The mixture was subsequently denatured at 70 ℃ and centrifuged to precipitate proteins. The determination, limit of copper (3σ) was 1.2μg ·L^-1 and the recovery was 92. 3%- 104%, and the determination. Limit of zinc(3σ) was 0. 098μg ·L^-1 and the recovery was 90%-107%. This two-step precipitation method can be used to determine non-protein-bound copper in tumor patients and healthy people.