Using the electrochemical deposition method, CdS thin films were deposited from acid solutions (pH = 2.5) containing CdSO4 and Na2S2O3 on indium-oxide coated glass substrates. These films were annealed in N-2, air, or O-2 atmosphere at 200-500 degrees C for 30 min. Photoluminescence spectra were measured at 77 K. For the films annealed in N-2, the band edge emission became weaker and the luminescence due to defects shifted to longer wavelengths as the annealing temperature was raised above 300 degrees C, However, for the firms annealed in air or O-2, the band edge emission was observed strongly irrespective of the annealing temperature and the luminescence due to defects was weak. Thus the O-2 annealing is useful for the defects reduction.