Visible light-responsive SnO2/g-C3N4 nanocomposite photocatalysts were prepared by ultrasonic-assisting deposition method with melamine as a g-C3N4 precursor. The as-prepared photocatalysts were characterized by X-ray diffraction, transmission electron microscopy, UV-vis diffuse reflectance spectroscopy, Fourier transform infrared spectra and photoluminescence emission spectra. The photocatalytic activities of the samples were evaluated by monitoring the degradation of methyl orange solution under visible light irradiation (wavelength a parts per thousand yen400 nm). The results show that the SnO2 nanoparticles with the size of 2-3 nm are dispersed on the surface of g-C3N4 evenly in SnO2/g-C3N4 nanocomposites. The visible-light photocatalytic activity of SnO2/g-C3N4 nanocomposites is much higher than that of pure g-C3N4, and increases at first and then decreases with the increment of the content of g-C3N4 in the nanocomposites. The visible-light photocatalytic mechanism of the investigated nanocomposites has been discussed.