Pretreated sugar cane bagasse (SCB) could be directly degraded by a designed coculture system with Clostridium thermocellum and Thermoanaerobacterium aotearoense for biological hydrogen and ethanol production, and the production was remarkably improved by CaCO3 supplementation. Here, the effects of CaCO3 concentration (10-100 mM) on the production of hydrogen and ethanol were investigated. Under the optimal CaCO3 concentration of 40 mM, hydrogen production reached 87.56 +/- 4.08 mmol/L from 2% pretreated SCB with a yield of 4.38 mmol H-2/g SCB, an 88.62% increase over the culture without added CaCO3 (46.42 +/- 1.22 mmol/L and 2.32 mmol H-2/g SCB). Additionally, the maximum ethanol concentration reached 10.60 +/- 0.81 mM, a 192.82% increase over the control (3.62 +/- 0.14 mM). The stimulatory effect of CaCO3 on the biodegradation of SCB was primarily ascribed to the buffering capacity of CO32-.This study developed a novel strategy to improve SCB biodegradation for biofuel production.