Human ribonucleotide reductase (RR) small subunits, M2 and P53R2, play key roles in forming RR holoenzyme and supplying nucleotide precursors for DNA replication and repair. Currently, we are studying the redox property, structure, and function of hRRM2 and p53R2. In the cell-free system, p53R2 did not oxidize a reactive oxygen species (ROS) indicator Carboxy-H 2DCFDA, but hRRM2 did. Further studies demonstrated that purified recombinant p53R2 protein has the catalase activity to scavenge H 2O2. Over-expression of p53R2 reduced intracellular ROS and protected the mitochondrial membrane potential against oxidative stress, whereas over-expression of hRRM2 did not result in the collapse of mitochondrial membrane potential. Our findings suggest that p53R2 may play a key role in defending oxidative stress by scavenging ROS, and this antioxidant property is also important for its enzymatic activity.