Three s-triazine/carbazole based bipolar molecules: 9,9′,9″-(1,3,5-triazine-2,4,6-triyl)tris(3,6-di-tert-butyl-carbazole) (TazTCz), 9,9′-(6-phenyl-1,3,5-triazine-2,4-diyl)bis(3,6-di-tert-butyl-carbazole) (PhTazDCz) and 9-(4,6-diphenyl-1,3,5-triazine-2-yl)-3,6-di-tert-butyl-carbazole (DPhTazCz) were synthesized and applied as host materials for blue phosphorescent organic light-emitting diodes (PHOLEDs). The triplet state energy levels are ~2.86 eV for these compounds, ensured their application in blue PHOLEDs. X-ray single crystal structural analysis revealed a non-planar structure of TazTCz, which prevented the intermolecular close-packing in the solid state. X-ray diffraction spectra also confirmed that TazTCz and PhTazDCz could form amorphous films with spin-coating method. The bipolar characteristics of as-synthesized compounds were confirmed by carrier mobility measurement. The device parameters of blue PHOLEDs based on these compounds as host materials exhibit that the balance of electron and hole mobilities is benefit for reducing turn-on voltage and efficiency roll-off at high current density.