The impact of particle size and internal grain boundaries of Beta zeolites was investigated in n-pentane isomerization over bifunctional Pt/Beta catalysts, by comparing the catalytic performance of four as synthesized Pt/Beta samples that possess an identical Pt loading (0.5 wt%), but use four distinct Beta zeolites. Three of them contain polycrystalline zeolites, consisting of nano-sized crystals with a similar size of 9-13 nm, but having different average particle sizes (i.e., 1340, 830, and 250 nm) and numerous internal grain boundaries, as found via high-resolution transmission electron microscopy. The last catalyst contains single-crystalline zeolite, with an average particle size of 225 nm, and no observed internal grain boundaries. At low reaction temperature (<578 K), the particle size and internal grain boundaries do not change the apparent activity, because activity is controlled by reaction on the acid sites of the zeolite. At high reaction temperature (>614 K), a large particle size and the presence of internal grain boundaries significantly reduce the apparent activity, because of the extended diffusion path and additional diffusion barriers, which are probably caused by a mismatch in micropore alignment and gas-zeolite interfaces at these grain boundaries. Due to the small particle size and absence of internal grain boundaries, the observed activity for single-crystalline Beta can be 60-212% higher than for polycrystalline counterparts, even though it possesses a much weaker intrinsic acidity. This shows, remarkably, that single-crystalline zeolites with less internal grain boundaries can achieve a much higher catalytic activity. (C) 2018 The Authors. Published by Elsevier Inc.