IrO2-SnO2 coatings with different Sn contents were prepared on titanium substrate by a thermal decomposition method. The relationship between the capacitance properties and microstructure was investigated by means of cyclic voltammetry, constant current discharge measurements and transmission electron microscope (TEM). Results show that the specific capacitance of IrO2-SnO2 coating electrodes increases first and then decreases with the Sn content increasing. The reversibilities and fast charging and discharging behaviors of the electrode materials are improved gradually. The maximal specific capacitance reaches 485.07 F/g when the content of Sn is 70 mol%, 7.5 times larger than that of pure IrO2 electrode. The coating is of amorphous structure, including the nanocrystalline with about 1 nm size and narrow size distribution, which increases the active site area.