This study suggests the best determining the tank size of rainwater harvesting system about an apartment house in the light of domestic weather characteristic, architectural characteristic, using characteristic of recycling rainwater, recently, as increasing awareness of eco-friendly building, though there are many introductions of environmental techniques making the best use of rainwater in domestic as alternative sources of shortage of water, in case of rainwater recycling facilities, it is under review only whether or not facilities application regardless of a size of building. as using of this study, it will be the fundamental data projecting the best the tank size of rainwater harvesting system in applying rainwater recycling facilities of an apartment building.