A series of experiments were carried out to investigate the influences of cold surface temperature, air relative humidity and air temperature on frost deposition on a horizontal flat copper plate in free convection. The plate was cooled by liquid nitrogen. The results show that the cold surface temperature has an essential influence on frost deposition mechanism. The frost deposition mechanism is different for different cold surface temperature levels. For ultra-low cold surface temperatures, the frost layer growth is outer boundary layer mechanism controlled. Under this mechanism, the frost layer is formed mainly by heavy phase drift that is formed in the vicinity of the cold surface and the frost layer growth rate is significantly smaller than that under ordinary-low temperature conditions. What is more important is that there is a tendency that the frost layer thickness decreases as the cold surface temperature decreases, which is totally different to the frost growth on the cold surfaces of ordinary-low temperature. The results also show that there is a transition temperature. For a cold surface whose temperature is lower than this transition temperature, the frost formation is outer boundary layer mechanism controlled. And this transition temperature is affected by air relative humidity and air temperature. Increasing air relative humidity for a given air temperature or air temperature for a given air relative humidity will accelerate the frost formation mechanism transition. (C) 2017 Elsevier Ltd. All rights reserved.