This study, with Hongdong Reservoir, is intended to evaluate the changes in water quality in the lake before and after rainfall in summer. Various non-point source pollutions are scattered around the reservoir, and to determine the pollution level by pollution source, samples were taken at the same point before rainfall (1st), during rainfall (2nd) and after rainfall (3rd) and concentration was measured. Pollutant concentration curve at the measuring points (HDS1, HDS2, HDS3) appeared to be similar with the hydrological curve. When comparing the concentration immediately before rainfall with event mean concentration (EMC), SS and COD were 4 - 59 times and 1 - 4 times, respectively. However, when it comes to total nitrogen (T-N), concentration arrived at the reservoir stayed the range of 1.3 - 12.0 mg/L in all 3 cases without significant variation, which indicated that total nitrogen load is critical when arriving at the reservoir, irrespective of rainfall, and thus it’s necessary to consider non-point source pollution runoff also in addition to point source pollution when developing the water quality improvement measures in reservoir.