Mesoporous-assembled TiO2 nanocrystals with very high photocatalytic H-2 production activity were synthesized through a modified sol-gel process with the aid of urea as mesopore-directing agent, heat-treated under various calcination temperatures, and assessed for their photocatalytic H-2 production activity via water splitting reaction. The resulting mesoporous-assembled TiO2 nanocrystals were systematically characterized by N-2 adsorption desorption analysis, surface area and pore size distribution analyses, X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The experimental results showed that the photocatalytic H-2 production activity of the synthesized mesoporous-assembled TiO2 nanocrystal calcined at 500 degrees C, which possessed very narrow pore size distribution, was extraordinarily higher than that of the commercially available P-25 TiO2 and ST-01 TiO2 powders. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.