Abstract: The window cracks is one of the important passageway that atmospheric fine particulate matter (PM _2.5) get through into the indoor environment and then causing the indoor personal PM _2.5 exposure. In order to evaluate the characteristics of air infiltration through the window cracks, a longitudinal monitoring regarding both indoor and outdoor PM _2.5 mass concentrations and meteorological parameters (e.g. temperature, wind speed and relative humidity) were carried out in an unoccupied office located in Beijing from September 2013 to August 2014, with the condition of no mechanical ventilation and no indoor PM _2.5 pollution sources. Additionally, a mathematical model of air infiltration rate based on a large number of measured data combined with mass conservation theorem and the method of mathematical statistics was developed. The results show that the outdoor weather conditions and atmospheric PM _2.5 concentrations is the important factor of the air infiltration rate in a room which has a certain window structure (the PM _2.5 penetration factor ( P) is 0.93±0.01 and deposition rate ( k) is 0.10±0.03 for the sampling site). Influenced by the change of the outdoor wind speed, the average air infiltration rate is about 0.10h ^-1 at static stability days, and it is about 0.22h ^-1 at gentle breeze days, as well as 0.39h ^-1 at moderate breeze days.