A method for the determination of sialic acids in serum with ultrafiltration-capillary electrophoresis is described and the operation was optimized. Sialic acids were directly separated and analyzed with UV detection at 195 nm and without pre-or post-column derivatization. The recovery of N-acetylneuraminic acid (NANA) was 92.6%, the concentration and mass detection limit of NANA were 9.6 mumol/L and 39 fmol respectively. This method was used for the determination of NANA level in the serum of 11 lung cancer patients and 30 normal adults. The results showed that the average concentration of NANA in the serum of patients was much higher than that of normal adults with P < 0.001. The results were also compared with those obtained by the traditional colorimetric method, with good linear relationship of r = 0.983 at n = 10. It is concluded that the method described in this paper is simple and sensitive, and is suitable for basic research and clinical applications to malignant tumors.