Co-combustion characteristics of two coals and two biomass are investigated through thermogravimetric method in O-2/CO2 atmosphere. Characteristic parameters of coal combustion deduced from TG-DTG curves show that, at the O-2 concentration of 20%, substitution of CO2 for N-2 brings about delay in ignition, exaltation in burnout, and drop in combustion integration parameter. To improve performances of coal combustion in O-2/CO2 atmosphere, biomass is added to form co-combustion. The difficulty of coal combustion in O-2/CO2 atmosphere can be effectively reduced at a co-combustion ratio of 20%. Activation energy during the coal combustion process is evidently reduced with the addition of biomass.