Magnetic tunnel junctions (MTJs) with structures of Ta(5 nm)/Cu(30 nm)/Ta(5 nm) /Ni79Fe21 (5 nm)/Ir22Mn78(12 nm) /Co62Fe20B18 (4 nm)/MgO( d) /Al(0.8 nm)-oxide/Co62Fe20B18(6 nm)/Cu(30 nm)/Ta(5 nm) on a thermally oxidized Si wafer substrate were fabricated by magnetron sputtering and photolithographic patteing method. The tunnel magnetoresistance (TMR) and the bias dependence of TMR at room temperature for the MTJs with different thickness d of MgO are investigated. TMR values of over 50% as high as those of the MTJs with pure Al-O barrier and the TMR-voltage curve asymmetries, which vary with the increase of d, are observed in the MTJs with hybrid barriers after annealing at 265℃ for an hour. The dependences of TMR, resistance and coercivity of the MTJs with composite barriers on temperature are also investigated.