The causes of the proton leak, its contribution to basal metabolic rate (BMR), influencing effect factors, and its physiological significance are reviewed. The mitochondrial proton leak occurs in intact cells and tissues, and divides energy into heat production and ATP turnover. The heat production can account for 20%～30% of BMR. However the ability to allow rapid switching of proton flux from leak to ATP turnover may have an even more important significance than that of heat production. The physiological functions of proton leak are including producing heat, increasing the potential for regulation of metabolism, reducing harmful free radical production and regulating carbon fluxes.