Mesoporous and large mesoporous SnO₂ powders were prepared by self-assembly of surfactants as a template and then their potentials as semiconductor gas sensor materials have been examined. Phosphoric acid treatment of as-prepared powders was found to be effective for suppressing the crystallite growth and then maintaining their ordered mesoporous and large mesoporous structure as well as large specific surface area up to elevated temperatures. Owing to their thermally stable ordered porous structure, mesoporous and large mesoporous SnO₂ sensors showed larger gas response than the sensors fabricated with conventionally prepared powder. Furthermore, simultaneous surface modification of conventional SnO₂ powder with Ru loading and subsequent coating with a mesoporous SnO₂ layer was proved to be an effective approach to improve the gas sensing properties. The improved response was considered to arise from a synergistic effect of the diffusion control by the mesoporous SnO₂ layer and the chemical sensitization by the Ru loaded.