Low temperatures during the booting stage reduce rice yields by causing cold-induced male sterility. To determine whether antioxidant capacity affects the ability of rice plants to withstand chilling at the booting stage, we produced transgenic rice plants that overexpress OsAPXa and have increased APX activity. The effect of increased APX activity on the levels of H(2)O(2) and lipid peroxidation were determined by measuring H(2)O(2) levels and malondialdehyde (MDA) contents in spikelets during cold treatments at the booting stage. The levels of H(2)O(2) and the MDA content increased by 1.5-fold and twofold, respectively in WT plants subjected to a 12 °C treatment for 6 days. In contrast, transgenic lines showed small changes in H(2)O(2) levels and MDA content under cold stress, and H(2)O(2) levels and MDA content were significantly lower than in WT plants. APX activity showed negative correlations with levels of H(2)O(2) and MDA content, which increased during cold treatment. Cold tolerance at the booting stage in transgenic lines and WT plants was evaluated. Spikelet fertility was significantly higher in transgenic lines than in WT plants after a 12 °C treatment for 6 days. These results indicate that higher APX activity enhances H(2)O(2)-scavenging capacity and protects spikelets from lipid peroxidation, thereby increasing spikelet fertility under cold stress.