Charge trapping in phosphorescent light emitting diodes was investigated by changing doping concentration of phosphorescent emitting material in light emitting layer. In red phosphorescent devices, both hole and electron trapping had the same effect on charge injection, while in green devices hole trapping played a major role in determining charge injection. In blue devices, hole or electron trapping effect was not observed. In addition, the efficiency of red and green devices could be improved by graded doping structure due to high luminance at the same driving voltage.