Using density functional theory and quantum transport calculations based on nonequilibum Green’s function formalism,we investigate the charge transport properties of endohedral M@C20 (M = Na and K) metallofullerenes.Our results show that the conductance of C20 fullerene can be obviously improved by insertion of alkali atom at its centre.Both linear and nonlinear sections are found on the Ⅰ-Ⅴ curves of the Au-M@C20-Au two-probe systems.The novel negative differential resistance behaviour is also observed in Na@C20 molecule but not in K@C20.