Self-ordered and structure-controlled transparent films of tin-modified mesoporous silica (Sn/Si ratio of 0.5-3%) were first prepared using a molecule surfactant template method employing spin coating. A surface photovoltage (SPV) NO2 gas sensor was then fabricated using these self-ordered tin-modified mesoporous silica thin films based on a metal-insulator- semiconductor structure. Highly sensitive tin-modified mesoporous silica was obtained that could detect NO2 gas concentrations of as low as 300 ppb at room temperature. The detection mechanism for NO2 is believed to involve both the surface area, which contributes to the change in dielectric constant, and the amount of tin incorporated, which contributes to the change in charge. It was found that, in this SPV sensor, the optimal Sn/Si ratio of 0.5% delivered record-high sensing performance.