Airborne ultrafine particles (UFP) have been associated with human mortality and morbidity. Human exposure to ambient UFP occurs indoors due to entry of UFP into buildings. This study investigates UFP infiltration as a function of building operating conditions such as central air distribution fan operation and window position. Experiments were conducted involving continuous monitoring of indoor and outdoor UFP concentrations along with air change rate measurement in a test house. The study results showed that the UFP infiltration varies with particle size, air change rate and central fan operation. Infiltration increases with particle size from 5 nm to 100 nm. Larger window openings lead to higher infiltration factors due to increased air change rates. Smaller values of infiltration factor were observed with the central fan on, which was likely caused by additional particle deposition loss to the furnace filter and duct surfaces.