Cyclic voltammograms were investigated to understand the electrodeposition behaviors of CuGaSe2 thin films. All experiments were carried out on ITO electrode to prevent evolution of hydrogen gas. Metallic gallium cannot be deposited from Ga unitary electrolyte. However, in the cyclic voltammograms of ternary Cu–Ga–Se system, a reduction peak at −0.6 V was observed with addition of GaCl3. It corresponds to the underpotential deposition (UPD) of gallium in the presence of selenium and consequently formed gallium selenides and cupper–gallium–selenium compounds. Gallium contents in the CGS films were increased with gallium concentration in the electrolyte increased. The highest and optimum contents of CGS film were obtained at −0.6 V in 5 mM Cu, 10 mM Ga, 10 mM Se.