Optical modeling based on the transfer matrix method is employed to investigate the performance of the organic planar heterojunction solar cell with rubrene/C70 as the active layer. The detailed investigation is directed into the effects of layer thickness of the rubrene and C70 on the total absorbed photon density in the active layer. It is revealed that the optical interference plays important role in the performance of the device and the optimal device performance is achieved when the thicknesses of the rubrene and C70 are set as 33 and 28 nm. The simulated results are also confirmed by the experimental data.