We have optimized and evaluated organic thin-film solar cell devices with a structure of graded junction. The graded junction consisting of donor and accepter materials was fabricated by varying the deposition rates of both materials with a continuous grading, using two evaporation sources of cupper phthalocyanine and fullerene as p- and n-type materials, respectively. By evaluating device characteristics, optimized device structure ITO/CuPc (10nm)/graded layer (35nm)/C₆₀ (15nm)/BCP (10nm)/Ag (100nm) with an efficiency of 1.36% was obtained. In the structure, short-circuit current density was the largest and existence of larger voltage dependence in current density was observed. In addition, we have measured temperature dependences of current density versus voltage characteristics in the graded organic solar cell under illumination. The carrier extraction was enhanced by changing voltage possibly due to the internal electric field of the graded junction.