We investigate the field electron emission for amorphous carbon (a-C) films deposited on Si (100) substrates through a niobium carbide buffer layer with different structures and find that the niobium carbide buffer layer can substantially improve the electron field emission properties of a-C films, which can be attributed to an increase in the enhancement factor β on the surface of a-C films after the insertion of the niobium carbide layer in between a-C film and substrate. Moreover, a phase transition for niobium carbide layer from hexagonal (Nb2C) to cubic (NbC) structure, revealed by x-ray diffraction, further enhances the electron field emission. The first-principles calculated results show that the work function of NbC is lower than that of Nb2C, which is the reason why the electron emission of a-C is further enhanced.