Pure sulfide Cu2ZnSnS4 (CZTS) thin films were fabricated on Mo-coated glass substrate by spray pyrolysis of an aqueous precursor solution containing Cu(NO3)(2), Zn(NO3)(2), Sn(CH3SO3)(2) and thiourea followed by sulfurization at 600 degrees C for 30 min. Cu composition was controlled by varying Cu concentration in the precursor solution for optimization. It was found that Cu composition affected to not only structure but also morphology of CZTS films. At an optimal composition, we obtained a CZTS-based solar cell with maximum conversion efficiency of more than 8 %.