To invest the luminescent characteristics of red light emitting OLED device, a dual dopant system was incorporated into the emitting layer. The multiple layer OLED device structure was $ITO(1500;{AA})/HIL(200;{AA})/a-NPD(600;{AA})/EML(300;{AA})/Alq_3(200;{AA})/LiF(7;{AA})/Al(1800;{AA})$. The concentrations of the rubrene dopant were tested at 0 vol.%, 3 vol.%, 6 vol.% and 9 vol.%. The maximum device efficiency and life time were obtained at the rubrene dopant concentration of 6 vol.%. Emission spectrum and color coordinate of devices showed no relationship with rubrene dopant concentration. Experiment results show that rubrene dopant absorbs energy from $Alq_3$ host and transfer it to RD1 dopant acting as an energy intermediate and influencing the device efficiency, finally the red light is emitted from the RD1 dopant.