The exciplex forming co-host with phosphorescent dopant system has potential to realize highly efficient phosphorescent organic light emitting didoes (PhOLEDs). However, the exciplex forming co-host for blue phosphorescent OLEDs has been rarely introduced because of higher triplet level of the blue dopant than green and red dopants. In this work, a novel exciplex forming co-host with high triplet energy level is developed by mixing a phosphine oxide based electron transporting material, PO-T2T, and a hole transporting material, N,N'-dicarbazolyl-3,5-benzene (mCP). Photo-physical analysis shows that the exciplexes are formed efficiently in the host and the energy transfer from the exciplex to blue phosphorescent dopant (iridium(III) bis[(4,6-difluorophenyl)-pyridinato-N,C2'] picolinate; FIrpic) is also efficient, enabling the triplet harvest without energy loss. As a result, an unprecedented high performance blue PhOLED with the exciplex forming co-host is demonstrated, showing a maximum external quantum efficiency (EQE) of 30.3%, a maximum power efficiency of 66 lm W-1, and low driving voltage of 2.75 at 100 cd m(-2), 3.29 V at 1000 cd m(-2), and 4.65 V at 10 000 cd m(-2), respectively. The importance of the exciton confinement in the exciplex forming co-host is further investigated which is directly related to the performance of PhOLEDs.