Two 0.5 wt % Pt/(zeolite beta-alumina) catalysts with zeolite crystal sizes of 200-500 nm (designated as Z1) and 10-30 nm (designated as Z2) showed similar acidities (ammonia temperature-programmed desorption), surface areas, mesopore volumes (N-2 adsorption), Pt-component hydrogenation activities, and coke contents after catalytic runs (temperature-programmed oxidation). The two zeolite components of these catalysts displayed the same framework structures (XRD, HRTEM, and FTIR spectroscopy). The nanocrystals of Z2 comprised individual single A (tetragonal) or B (monoclinic) polymorphs of zeolite beta, whereas the same polymorphs existed in the microcrystals of Z1 as domains of similar to20 nm that grow simultaneously, side by side, within the common grain. In the hydrocracking of heavy vacuum gas oil (HVGO) at 623 K and 5.5 MPa, the decrease in zeolite crystal size from the upper to the lower limit of the nanoscale range (i.e., from Z1 to Z2) gave an approximately 2-fold increase in both the rate constant and the effectiveness factor. The critical crystal size of zeolite beta beyond which the diffusion limitations became insignificant was 50 nm. The effective diffusion coefficient of HVGO inside the crystals of zeolite beta was 2 x 10(-14) cm(2)/g. The kinetic measurements and Thiele modulus analysis provided evidence for the significant contribution of the internal surface of zeolite channels to the catalytic process.