Abstract: The present paper has described the determination of average particle size by projection area method, i.e.a beam of parallel light was passed through the powder with given weight and density which was distributed in liquid medium. The photochopping area was measured by projection area apparatus, so that the average powder particle size was enabled to determine. The structure of apparatus has been illustrated. Experimental results have shown that the results obtAlNed by this method are similar to those by Fisher method and exhibit superior reproducibility.