We propose a new coating technique for fused silica capillaries using silanization with trimethylchlorosilane and diethylamine as a mediating agent in DNA separation using capillary electrophoresis. The proposed coating technique is simple and stable at a high pH. Capillaries coated by the new preparation method give excellent reproducibility for DNA fragment analysis with a good relative standard deviation of less than 0.7% for 150 runs and good stability at pH 8.2.