Sensing performance of mixed-potential gas sensors is highly dependent on the electrode microstructure. To shed light on the true sensing behavior, very thin electrode with well-defined microstructure is desirable. This work fabricated SnO2 porous thin films on YSZ (yttria-stabilized zirconia) substrate by using a monolayer polystyrene sphere template method, and obtained three planar sensors by varying the template diameter. The thin films were characterized by a periodically ordered inverse opal structure and a thickness of 121−448 nm. As the template diameter increased, the thin-film sensors exhibited decreasing response to H2 but similar response time and selectivity. In comparison with regular thick-film sensors, the thin-film sensors had lower H2 sensitivity and selectivity but a faster response speed particularly at lower concentrations. Catalytic conversion measurements revealed a significant H2 oxidation activity of SnO2. The microstructure-dependent sensing behavior was discussed in relation to the effective H2 concentration and variation of TPB density.