By using the direct solar radiation data measured by a CE-318 sun photometer in Inner Mongolia during the period between June 2002 and May 2003,the aerosol optical thickness(AOT) and size distribution were retrieved by extinction method.Results show that there were significant variations in temporal-spacial distributions of AOT.The largest mean AOT appeared in spring[AOT(λ=440nm)=0.78] with the least in winter[AOT(λ=440nm)=0.13].AOT was always largest in Ejin Qi and least in Dongsheng.There were four patterns of day-time variations:(1)AOT is high in the morning and low in the evening;(2)AOT is low in the morning and high in the evening;(3)AOT peaks at noon;(4)AOT varies gently.This is mainly associated with dust weather,turbulence of boundary layer and human activities.The aerosol size distribution basically satisfied the Junge distribution and appeared an obvious peaks at r =(0.3μm),0.6μm,and 1.0μm.However,it shows a significant discrepancy under different weather conditions.Large dust particles and giant dust particles remarkably increased in dust weather.The aerosol number concentration in dust weather was about one order of magnitude larger than that in clean weather.And it was also larger in spring than that in winter,but the both were still within the same order of magnitude.