Ag2S thin films were fabricated on the ITO-coated glass substrates by cathodically electro-deposition from the mixture solution including 0.01mol·L-1 AgNO3 and 0.05 mol·L-1 Na2S2O3 with pH=2.5 at room temperature. The microstructure and surface morphology of the films were investigated with the deposition potential (E) varied from -0.23V to -0.28V. The X-ray diffractograms show that the deposited films are monoclinic Ag2S with the relative deviation of cell parameters within 1.5%. The estimated cell parameters of the Ag2S films deposited at E = -0.25V are closest to those of the standard sample. The SEM pictures show that films are uniform with better compactness at more negative deposition potential, but there are some aggregation when the potential up to -0.28V. According to the AFM images, the root mean square (RMS) roughness and grain size decrease with the decreasing of potential absolute value, but they will increase when the deposition potential is too small. The best potential is -0.25V for depositing Ag2S thin films.