In urban areas, there is concern that storm water runoff and flooding may be intensified because of the effect of climate change on precipitation amounts, intensities, and frequencies. The objective of this study is to use the Storm Water Management Model (SWMM) to assess the existing drainage’s capacity, to quantify the impact of climate change on existing drainage system and to propose a structural measure for reducing the flood impact. The SWMM was set up to simulate the single event that generated into hyetograph with 6mn time step by Soil Conservation Service (SCS) curve type II in year 1990 (current extreme rainfall over 20 years), 2014, 2055, 2 year and 5 year design strom located in Chamkamorn district, the highly urbanized area in Phnom Penh. The results indicate that more surface flooding will be expected in the future. The number of critical nodes in the system and the number of pipes performing over their capacity also will increase due to the greater expected rainfall. The highest flood is in 1-day maximum rainfall for a 20-year centered on 2055 using an Representative Concentration Pathway (RCP 8.5) with maximum flooding volume 153212m3 and duration of flood is 2h 27mn. In 5 different scenarios, increasing percentage of highest inflow was occurred in junction14 (J14) comparing to its flow capacity. The result indicated that the highest increasing is on the projected 1 day of maximum rainfall for a 20-year period centered on 2055 using an RCP 8.5 was increased 949%. Based on Japan International Cooperation Agency (JICA) report aimed to reduce flooding within 2hours so the diameter has been enlarged for year 2055 for conduit C16 from 1.5m to 1.8m that can reduce from 2h 27mn to 1h 10mn.