Efficient blue and white phosphorescent organic light-emitting diodes (PhOLEDs) with low turn-on voltage were fabricated by using mixed hole and electron transporting layer materials as the co-host in the emissive layer. The optimized performance of blue PhOLEDs was achieved with the current efficiency, external quantum efficiency and power efficiency are 45.11 cd/A, 19.21% and 45.67 lm/W, respectively. Based on the blue-PhOLEDs, efficient white PhOLEDs were achieved with ultrathin green and red emitters were respectively inserted into different locations of the emitting layer. The influences of the locations on the emissive spectra of white PHOLED were investigated. The optimized white PHOLED with the maximum power efficiency of 45.22 lm/W and the peak external quantum efficiency of 16.39% were achieved.