A convenient and valid method for the determination of ascorbic acid(AA) and dehydroascorbic acid(DHAA) in plasma and dialysate from patients with uremia by high performance liquid chromatography with electrochemical detection is described. A mixture of 0.8 g/L metaphosphoric acid and 18% (volume fraction) perchloric acid was used as a protein precipitant and the extractant for AA from biosamples. It was also a good stabilizer for AA in samples. The proposed method is satisfied for routine screening of vitamin C in clinical applications with a correlation coefficient of more than 0.99 in the range of 2 mumol/L-40 mumol/L AA. The within-day precision was less than 8.9% and 10.55% for AA and DHAA, respectively. The recoveries of AA in plasma and dialysate were over 95% and 78%, respectively.