An indirect square polarographic method was established f or the determination of ascorbic acid in pharmaceuticals using the oxidation-re duction action between Cu2＋ and ascorbic acid. There was a reduction wav e of Cu2＋ at about －0．24　V in buffer solution of NH3-NH4 Cl. The quantities of ascorbic acid is determination according to the decrement of peak heights when ascorbic acid was added. The linear range for ascrobic acid was 2.0×10－6～3．0×10－4 mol/L. Ascorbic acid in Vitami n C tablets was determined, the relative standard deviation is 2.9%.