Since spray combustion is widely used in the combustor, scientific understanding of spray combustion is essential for more efficient energy conversion. Single droplet combustion is considered as the simplest model of spray combustion and it is known that the square of droplet diameter, d^2, decreases almost linearly and the decrease rate (i.e. burning rate constant) is constant in vaporization process. In this study, effects of ambient composition and pressure on the burning rate constant of single ethanol droplet were experimentally investigated under microgravity. At 0.5MPa of ambient pressure or 60% of carbon dioxide concentration in the ambient gas, d^2 does not decrease linearly and soot was observed in the flame image. It is expected that the formation of the soot influences the unsteady behavior of burning rate constant.