Several sorts of factors affecting to the self-purification of a stream were investigated in the main stream of Han River together with a tributary of Wangsukcheon during the spring season of April and May, 1978. The BOD_5 concentration at site 1, 3 and 4 was 2.82㎎/ℓ, 3.02㎎/ℓ and 2.98㎎/ℓ, respectively. The BOD reaction rate was 0.203/day, 0.201/day and 0.221/day, respectively. And the same value of 0.601/day was identified as reaeration rate at three sites. General water quality analysis was carried out from the immediate vicinity of confluence of Wangsukcheon to three kilometers below with several interim blocks to determine the critical point. The site which showed highest degree of pollutant concentration was cited as critical point. Results obtained exhibited a poor mixing of polluted water from Wansukcheon with the main stream of Han River water which makes near to impossible to apply the Streeter-Phelps equation used for the simulation of pollutants in a stream within the study block of Han River. The result also showed that the population density of general bacteria and total biomass quantity was directly proportional to the concentration of BOD_5 and the deoxygenation rate had nothing to do with any of them.