Sand-dust aerosol particle size distributions have been measured by the APS-3310A(made by U.S.A) in Bayinhaote,Yanchi,and Yinchuan nearby the Helan Mountains during the April and May of 1998,and the April of 1999,respectively.A lot of data have been collected under different weather conditions,i.e.background,floating dust,blowing sand,and sand storm.The rules of sand aerosol particle size distribution are found through statistical analysis.Sand aerosol concentration is changeable in different weather processes.During the blowing sand and sand storm weather,aerosol concentration changes a lot,but during the floating dust weather,the circumstance don’t exist.The stronger the aerosol concentration,the more the number of aerosol particles greater than 2.5μm,and the more obvious the change in the concentration of various diameter ranges.During the four processes,coarse and fine particles have different contribution to the surface concentration.The mean instantaneous measured size distributions of sand aerosols are characterized by 1-mode-a single peak mode,which can be fitted very well by a log-normal distribution function.