Recently electret filter media are extensively used to improve indoor air quality. However, they are significantly affected on environmental conditions such as humidity, temperature and chemical substance. This study reported the effect of humidity and temperature on the surface electric potential, filtration efficiency and pressure drop of electrically charged fiber (ECF) composites. The surface electric potential and filtration efficiency of ECF composites decreased rapidly, but pressure drop increased slightly with increasing relative humidity (RH) and temperature conditions. Effect of temperature change was more significant than RH change. Relationship between filter efficiency and surface electric potential relationship of ECF composites exhibited a quadratic function.