Flow and pollution load were monitored at 2 combined sewer outlets (C-1 and C-2) of urban watersheds during dry weather from September, 2004 to April, 2006 for 20 months. The objectives were to investigate the diurnal variation of flow and pollutant load and to find the proper sampling time that could measure representative flow and pollutant load. Pollution load closed to the average daily load at C-1 could be measured at 00:00 hour and by the mean of 15:00 and 18:00 hour measures, and 15:00 and 21:00 hour measures, respectively. In addition at C-2, it was 21:00 hour and the mean of 15:00 and 18:00 hour measures. This study concluded that arbitrary sampling of flow and water quality could cause large errors in the estimation of urban pollution load and recommended that urban combined sewers should be monitored when flow and water quality showed daily average and concentration.