The hole blocking characteristics of 4,4',4 " -trifluoro-triazine (tfTZ) were studied. The current density vs. voltage characteristics and the electroluminescence spectra of elelctroluminescent(EL) devices, which were fabricated with the structure of ITO/tfTZ/hole transport layer/emitting layer were measured. In this letter, we investigated the electrical and optical effects accompanied by the thickness change of tfTZ layer. In EL devices including tfTZ layer, current density and turn-on voltage decreased in comparison with those without tfTZ layer.