We report the synthesis of mesostructured Pt films with extralarge periodicity from lyotropic liquid crystals consisting of block copolymers (polystyrene-b-polyethylene oxide, PS-b-PEO) on Au substrates by electrochemical deposition. The Pt films with three types of (two-dimensional (2D)-hexagonal, lamellar, and cage-type) mesostructures are successfully synthesized by controlling the compositional ratio between block copolymers and Pt species in precursor solutions. The mesostructured Pt films have high electrochemically active surface areas. The bumpy mesopore surfaces, which reflect the mesopore walls consisting of connected nanoparticles, greatly contribute to the enhancement of the surface areas. The mesopore walls have single crystal domains over 400 nm(2) region proved by the lattice fringes of Pt extending over several nanoparticles.