Amorphous MnO2/CNTs-OH (a-MnO2/CNTs-OH) was prepared by direct reaction method and then heat treated at 400 degrees C under an argon atmosphere. The morphology characteristics indicate that MnO2 is amorphous and highly dispersed. The X-ray diffraction and X-ray photoelectron spectroscopy reveal that the existent form of manganese is a-MnO2. The oxygen reduction reaction performance investigated using a rotating disk electrode shows that the initial reduction potential of a-MnO2/CNTs-OH and a-MnO2/C are 0.905 and 0.844 V (vs. RHE). The Tafel slopes are 88.9 and 127.35 V dec(-1), respectively. The electron transfer numbers are 3.92 and 3.8, respectively, corresponding to a four-electron process. The stability experiments of liner sweep voltammogram show that a-MnO2/CNTs-OH decreases by 0.155 mA cm(-2) after scanning for 200 cycles at the rotating rate of 1500 rpm with a sweep rate of 5 mV s(-1). In I-t curves, the oxygen reduction current density of a-MnO2/CNTsOH and a-MnO2/C decreases by 19.69% and 34.7% after 60 h. Single cell tests of magnesium-air fuel cells show that the peak power densities of a-MnO2/CNTs-OH and a-MnO2/C are 79.2 and 62.4 mW cm(-2), respectively, and a-MnO2/CNTs-OH catalyst can continuously discharge for more than15 h at 50 mA cm(-2).