The photoelectrochemical properties of CuGaSe2 modified by deposition of a thin CdS layer were investigated. The CdS layer formed a p-n junction on the surface of the electrode, improving its photoelectrochemical properties. There was an optimal CdS thickness because of the balance between the charge separation effect and light absorption by CdS. CdS-deposited CuGaSe2 showed high stability under the observed reaction conditions and evolved hydrogen continuously for more than 10 days.