In order to characterize the particle shape quantitatively, six shape factors are calculated using particle geometrical parameters determined by the analysis of microscopic images. Three of them, i.e. short/long ratio (SM), roundness (R) and concavity (CA), are able to describe the particle shape of fluidized catalytic cracking (FCC) catalysts in three principal aspects. Mean shape factors and shape factor distributions are proposed to characterize the particle shape of FCC catalyst powders. It is proved by the two.-sample Kolmogorov-Smirnov test that there are few differences between shape factor distributions generated from about 500 particles and those obtained by counting more than 2000 particles. Satisfactory results are obtained with this method in industrial applications.