Aldehyde dehydrogenase (ALDH) isozymes of human erythrocyte were separated and purified. ALDH having low K_m value (74.6 μM) and high K_m value (174 μM) for acetaldehydde were named low K_m, ALDH and high K_m ALDH respectively. For low K_m, ALDH, both NAD^+ and NADP+ could be used as coenzyme, but high K_m ALDH used only NAD^+. Their molecular weights were determined according to pore-gradient polyacrylamide gel electrophoresis and found that low K_m ALDH was 269,000 dalton and high K_m, ALDH 273,000 dalton. SDS-polyacrylamide gel electrophoresis showed that they were both homotetramer, composed of four identical subunits. The molecular weight of subunit of these two isozymes were 59,500 and 60,600 dalton respectively. Both, high K_m and low K_m ALDH showed esterase activity as well as dehydrogenase activity. They were very stable at 35℃ and optimum pH of both isozymes were 9.5. isoelectric point of low K_m ALDH was 5.5.