The investigated sector of self purification capability is the mid and up stream of Jungryang main stream which length is 6. 5 km (from the confluence of Backsuk stream and the main stream to the confluence of Dobong stream and the main stream) The self purification capability evaluated as two cases, they are untreated and treated through Eujungbu city sewage treatment facilities, DO model of Streeter and Phelps. The results are as follows, The K₁ value in Jungyang stream is 0.216 day^(-1) (20℃), K₂ value is 4. 050 day^(-1) (20℃) by Langbien and Durum equation and f value is estimated 18. 75 which appeared higher than the value in the unpolluted streams. The assimilative capability evaluated by DO model of Streeter and Phelps is approximately 21 percent of self purification ratio in the main stream, which length is 6. 5 km from the confluence of Backsuk stream and the main stream to the confluence of Dobong stream and the main stream. In the case of treated by Eujungbu city sewage treatment facitities is assimilative capability. By DO model of Streeter and Phelps, the evaluated value has approximately 50% of self purification ratio and DO value will be increased to 5. 3 ㎎/ℓ from 1. 37 ㎎/ℓ in the untreated of Eujungbu city sewage treatment facilities.