The effect of particle size in the range of 0.16-2.01 mum on the hydrothermal stability of H-Y zeolite under hydrothermal treatment conditions similar to those used in FCC processes was investigated. The average particle size of 0.45 mum was found to be the optimum particle size of H-Y zeolite to retain a high percent crystallinity upon hydrothermal treatment. A new correlation was developed by plotting the relative crystallinity (C/C-0) against the reciprocal of the square root of the particle size (d(0)) of Y zeolite. The correlation accurately predicts the hydrothermal stability of Y zeolite in small to medium particle sizes for a given temperature. For the larger particle sizes of Y zeolite (>0.45 mum), no exact correlation between particle size and hydrothermal stability was found, probably because of the presence of more structural defects in the zeolite, as shown by higher Bronsted/Lewis acid ratios.