OBJECTIVE: The impairment of circadian clocks is a cause of obesity, but the pathophysiological role of the circadian clock in brown adipose tissue (BAT), a major tissue regulating energy metabolism, remains unclear. To address this issue, we investigated the effects of complete disruption of the BAT clock on thermogenesis and energy expenditure. METHODS: The mice with brown adipocyte-specific knockout of the core clock gene Bmal1 (BA-Bmal1 KO) were generated and analyzed. RESULTS: BA-Bmal1 KO mice maintained normal core body temperature by increasing shivering and locomotor activity, despite the elevated expression of thermogenic uncoupling protein 1 in BAT. BA-Bmal1 KO disrupted 24 h rhythmicity of fatty acid utilization in BAT, and mildly reduced both BAT thermogenesis and whole-body energy expenditure. The impact of BA-Bmal1 KO on the development of obesity became obvious when the mice were fed a high-fat diet. CONCLUSIONS: These results reveal the importance of BAT clock for the maintenance of energy homeostasis and the prevention of obesity.