We fabricated the thin films by RF sputtering from Na-mixed CuInSe2 target, and investigated the effect of the substrate temperature on the formation of CuIn3Se5 thin films. From electron probe microanalysis (EPMA) measurements, the copper content was found to decrease with increasing the substrate temperature, and the band gap of the thin films became large, suggesting the formation of CuIn3Se5 structure. The reduction of copper content was presumed to be due to the surface reaction during the growth. Based on the results, one of the growth models for CuIn3Se5 is proposed.