In the mist assisted film cooling, water droplets are injected along with the coolant air. The concentration difference between the water droplets and the coolant air results in the evaporation of the former leading to drop in temperature and enhancement of the cooling efficiency. In the present work, a new definition for mist-air film cooling effectiveness is proposed and the performance of mist assisted film cooling is investigated on the wall of straight channel in the presence of a film cooling hole. Detailed two dimensional computational studies are carried out for various droplet diameter, relative humidity of air and mist concentrations. Results show that the film cooling effectiveness is always larger for the case of mist-air compared to that of the dry air. Further, the film cooling effectiveness increases with the percentage of mist in the mist-air system. At a specified mass flow rate of dry air and various quantities of mist, the increase in droplet diameter results in the decrease of cooling effectiveness.The cooling effectiveness increases with the relative humidity of air at all percentages of mist. © 2018 Elsevier Ltd