Dissolved oxygen, BOD and flow pattern of the Nakdong River stream were measured for 3 years from 1978 to 1980 at quarterly intervals of January, April, June and October at 12 sites along the main stream from Sangju to Imhaejin and at 2 sites of Geumho and Nam River tributaries. With these data, the self-purification factors of the river were computed to obtain the following results: 1) The average BOD loads per day at the tributary of Geumho River were 94 tons in January, 39 tons in April, 60 tons in July and 54 tons in October, and these are considered to be the main source of water pollution toward the main stream of the Nakdong River. 2) Self-purification factors for the Hwawon-Hyunpung region of the main stream after receiving Geumho River water were computed to give $0.21{sim}0.59$ of deoxygenation $constant(K_1)$ and $0.56{sim}2.27$ of reaeration $constant(K_2)$. The oxygen-sag curves constructed for the main stream showed a remarkable decline at Hwawon and a quick recovery at Hyunpung, indicating a rapid decomposition of pollution loads received from the Geumho River. It was confirmed that the self-purification capacity of the Nakdong River was relatively high.