The study was conducted to evaluate the determinants of mass concentration of indoor particulate matter in a nursing home located in Taichung, Taiwan. PM2.5, PM10, temperature, relative humidity, CO, CO2, O3 and colony counts were collected in 2 bedrooms and their adjacent outdoor environments from November 2009 to January 2010. The results of multiple regression analysis suggested that the explanatory variables which included outdoor particle concentrations, indoor occupancy, different types of activities and ventilation accounted for 40.9% and 63.4% of the variance in the indoor PM2.5 concentration in Room A which is close to neighboring buildings and Room B which is close to main traffic, respectively. The explanatory variables accounted for 49.1% and 85.5% of the variance in the indoor PM10 concentration in Room A and B, respectively. Moreover, the result of correlation analysis showed that both indoor PM2.5 and PM10 concentrations were correlated to temperature, relative humidity and CO.