Abstract: The prevailing wind principle has long been adopted by urban planners, especially in the master plan stage. According to the principle, districts of industrial plants emitting air pollutants are located on the leeward of the city by prevailing wind, so that the urban air will not be polluted. But practi ce has proved that the principle is not alwaystrue. In this paper,the weakness of the prevailing wind principle is exposed and a new approach to the problem is proposed.It is pointed out that for the wind from any direction what determines the frequency of the occurrence of air pollution on the leeward of a given emissionsource is not the frequency of the wind but the frequency of tie occurrence of the weather conducive to air pollution associated with the wind. And the fact that the frequency of the prevailing wind is higher than that of the wind from other direction does not necessarily mean that the frequency of the occurrence of air pollution associated with the prevailing wind is higher than that associated with the wind from other direction.Therefore it is important for urban planners to find out the frequencies of occurrence of weather conducive to air pollution associated with winds from different directions. This can be done by using two. equations proposed in this paper. One is used to calculate the Wind Pollution Indeox by using routinely availabe meteorological data, which describes the level of air pollution with a given amount of air pollutants under the weather associated with the wind observed. The other is used to calculate statistically the Wind Pollution Frequency by using the data of Wind Pollution Idex, which describes the frequency of the weather conducive to air pollution associated with the winds from a given direction.