Objective： Hydrogen peroxide is produced endogenously and can cause mitochondrial dysfunction and metabolic compactions by inducing oxidative stress. Catalase is an antioxidant enzyme, which converts hydrogen peroxide into water and oxygen. However, few studies exist on the catalase KO (CKO) mice mechanism underlying the actions of catalase in obesity. Here, we show that ablation of catalase promotes diet-induced obesity by oxidative stress and mitochondrial dysfunction Methods： Seven-week-old WT and CKO mice were individually housed under a constant temperature (25OC) and 12-h light/dark cycle and subsequently randomly divided into two groups, a normal diet (ND, n = 10) and high fat diet (HFD, 60% of kcal from fat, n = 10), respectively. Mice were individually caged during experiment and fed ad libitum with free access to tap water for 4weeks. All values are expressed as mean ± SEM. Statistical analysis was performed by using SPSS. Results： CKO mice significantly increased body weight after the 4 weeks of HFD feeding and food efficiency ratio. CKO mice also significantly increased the weight of all white adipose tissue depots (epididymal, subcutaneous, perirenal and retroperitoneal) compared with the WT-HFD group. HFD feeding markedly increased the hepatic and epididymal fat H202 content in CKO mice. On the contrary to this result, the protein expression of the mitochondrial biogenesis factors PGC-lα, pAMPKo, TFAM and NRF-1 of liver and epididymal fat were significantly decreased in HFD fed CKO mice. Genes related Fatty acid oxidation and TCA cycle, CPTla, CPTlb, CPT2, Acoxl, Fgf21, Lead, Mead, Cs, Aconitase, IDH, SDH and MDG were down-regulated in the CKO mice. Furthermore HFD feeding led to reduce consumption of 02 and C02 and energy expenditure during both the light phase and dark phase in CKO mice. Conclusion： Ablation of catalase predisposes to diet-induced obesity through mitochondrial dysfunction by oxidative stress due to increased H202 content in liver and epididymal fat.