For the purpose of reducing occupants’ exposure to micro-particulates, air filtration has been widely used in air-conditioned buildings. This paper proposes a practical procedure to evaluate the filter performance of window-typed air-conditioners on removing micro-particulates (PM2.5). Five residential apartments fitted with air-conditioners in the bedrooms were investigated. The site investigation involved, operating an air-conditioner at two different settings of normal filtration; additional filtration scenarios were operated for comparison, as well as one off-mode measurement. During the measurement, concentrations of particulates and tracer gas were monitored simultaneously. Based on the decay profiles of PM2.5 and tracer gas, the performances of air-conditioners at normal and additional scenarios monitored in the five apartments were within the ranges of 1.7–3.0% and 11.9–23.3%, respectively. The performance of additional filters was about 7.6 times more effective than the normal filtration.