Generally, rainwater harvesting has been less acceptable as a stable water resource for the countries with large disparities in seasonal rainfall including Korea. In this study, rainwater harvesting system suitable for South Korea was investigated systematically. The simulation method utilizing daily rainfall data collected for the last 10 years was developed to determine the critical size of rainwater tank which could supply harvested rainwater continuously. The estimation of rainwater tank size was further simplified by developing a correlation of volume to area fractions. Extensive simulation was performed to evaluate the feasibility of rainwater harvesting for seven major cities in South Korea. The resulting rainwater tank was too large to be economically feasible, compared to daily harvested rainwater used. Thus, rainwater harvesting system was integrated with alternative water resources such as wastewater reclamation by membrane bioreactor (MBR) to enhance the applicability of rainwater harvesting without compromising continuous water supply.