We report narrow band gap bipolar host materials, CbPr-3 (9,9'-[(3,3'-Biphenyl-3.3'-yl-bipyridine)-1,3-biphenyl]bis-9H-carbazole) and Bim-4 (9,9'-[5-(1-phenyl-1H-benzimadazol-2yl)-1,3-phenylene] bis-9H-carbazole), for blue phosphorescent OLEDs application. These two bipolar hosts have high triplet energy of > 2.9 eV, capable of reducing the driving voltages and improving efficiencies. Significant low driving voltages of 7.4 and 6.6 V were obtained for CbPr-3 and Bim-4 hosts, compared with 9.0 V of the commonly used host, mCP (1,3-bis(9-carbazolyl)benzene). At a given constant luminance of 1000 cd/m2, the power efficiency of both the bipolar host devices was enhanced by 2.5 times.