The effects of metal-insulator interfacial roughness, modulated by Ar+ irradiation, on bias dependence of tunnel magnetoresistance (TMR) and electrical transport of CoFe–AlOx–CoFe magnetic tunnel junctions (MTJs) have been studied. Reduction of TMR ratio and asymmetric TMR falloff curves as a function of dc bias have been observed for Ar+-irradiated MTJs. The results are analyzed by x-ray reflectivity together with complex impedance techniques, indicating interfacial roughness which likely results in a proportional rising trap state density (TSD). Increasing TSD for Ar+-irradiated MTJs increases an unpolarized current which decreases TMR ratio. The asymmetric TMR falloff curves are attributed to the different TSDs of bottom and top CoFe–AlOx interfaces in tunneling process.