In many circumstances, it is infeasible to simulate the daily water yield in every land use or soil type of the watershed or river basin. These situations can be simulated in“Soil and Water Assessment Tool”(SWAT) using a concept called“hydrologic response units”(HRU's) within a topographically-defined subbasin. Soil water balance, crop growth, nutrient cycling management, etc., are simulated for each HRU. For the watershed of Bok-ha river, the effect of HRU's in SWAT has been studied in respect to water yield. The optimum number of HRU's was 23 based in data capacity and correlation coefficient.