Small molecule organic solar cells (OSCs) with the structure of indium tin oxide (ITO)/molybdenum trioxide (MoO₃) (5 nm)/rubrene (x nm)/fullerene (C₇₀) (y nm)/2, 9-dimethyl-4, 7-diphenyl-1, 10-phenanthroline (BCP) (6 nm)/aluminum (Al) (150 nm) are fabricated. The thickness of active layer for the devices is investigated in details. The results show that the optimum thicknesses of rubrene layer and C₇₀ layer are 30 nm and 25 nm, respectively. The degradation of the device is also investigated. The result indicates that the open-circuit voltage (V_oc) does not change, while the short-circuit current density (J_sc), fill factor (FF) and power conversion efficiency (PCE) decrease continuously with time. The degradation can be attributed to the oxygen in ambient diffusing and infiltrating into the active materials and reacting with C₇₀ in cells, which can result in the increase of interfacial series resistance.