Crystalline mesoporous Ta2O5 has been successfully synthesized by a one-pot route using P-123 as the structure directing agent (SDA). A series of crystalline mesoporous Ta2O5 samples has been prepared by changing the calcination temperature. The surface area decreased and the pore size increased with the increasing calcination temperature, which were the results of crystallite growth. At the same time, the pore volume was well maintained, which means limited shrinkage during the calcination of elevated temperature. The porous structure and crystal structure of as-synthesized mesoporous Ta2O5 were characterized by XRD, TG-DTA, SEM, TEM, and N2 sorption techniques. The photocatalytic activity of the as-synthesized mesoporous Ta2O5 with the cocatalyst NiOx for overall water splitting under ultraviolet (UV) light irradiation was systematically evaluated. The photocatalytic activity of crystalline mesoporous Ta2O5 showed about 3 times that of commercial Ta2O5 powder and 22 times that of amorphous mesoporous Ta2O5.