 Nanoscale manganese dioxide particles supported onto carbon nanotubes (MnO2/CNTs) were prepared through a microwave-assisted reduction method and used as a catalyst for oxygen reduction reaction (ORR) in an alkaline environment. The MnO2 with high crystallinity and nanosized birnessite-type was obtained by microwave irradiated for 60 s repeating 3 times. Especially, the MnO2/CNTs exhibited a positive onset potential (− 8 mV vs. Hg/HgO) of ORR, which was similar to that of 20 wt% Pt supported on CNTs (Pt/CNTs). It was also found that the Koutecky–Levich plots of MnO2/CNTs suggested that its ORR proceeds via a 4-electron pathway. In addition, the accelerated durability tests indicated that the MnO2/CNTs composite catalyst was more stable than Pt/CNTs in 0.1 M KOH solution. The enhanced electrochemical activity and stability of MnO2/CNTs composite electrocatalyst can be attributed to the high electrical conductivity of CNTs and unique microstructure between MnO2 and CNTs, making it a promising candidate as an oxygen reduction catalyst in metal–air batteries and fuel cells.