In this paper, the effect of a nitrogen-doped (N-doped) LaB₆ interfacial layer (IL) on p-type pentacene-based OFET was investigated. The pentacene-based OFET with top-contact/back-gate geometry was fabricated. A 2-nm-thick N-doped LaB₆ interfacial layer deposited on an 8-nm-thick SiO₂ gate insulator. A 10-nm-thick pentacene film was deposited by thermal evaporation at 100°C followed by Au contact and Al back gate electrodes formation. The fabricated OFET showed normally- off characteristics and a steep subthreshold swing (SS) of 84 mV/dec. from I_D-V_G and I_D-V_D characteristics. Furthermore, the aging characteristics of 6 months after the fabrication were investigated and it was found that V_TH and SS were stable when the N-doped LaB₆ IL was introduced at the interface between SiO₂ gate insulator and pentacene.