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Freezing processes of water and peanut oil droplets on a cold surface are investigated in this paper. We observed during our experiments that the base surface of a water droplet that is in direct contact with the cold surface keeps its original shape, but the other part of the droplet shows an obvious growth along the direction normal to the base surface. One small protrusion appears on the top of the water droplet at the end of the freezing process. The experimental observations also show that no obvious shape change happens during the freezing of peanut oil droplets. It is postulated that the effects of surface tension and volume dilatation resulted from liquid-to-solid phase change cause the shape change and protrusions formation. Based on this postulation, a physical and mathematical model is developed. The results of the model of a water droplet's freezing process correspond with our experimental observations. The observed phenomenon that frost-growth speed on the protrusion is higher than that on the other part of the water droplet is also analyzed.Frost formation is a very common phenomenon in cryogenic, refrigeration, and air conditioning engineering. The temperature of heat exchange units will undergo a gradually decreasing process during their thermal defrosting process. When humid air comes in contact with a cold wall whose temperature is below the dew point of air, vapor contained in the humid air will condensate and deposit on this cold surface in the form of small water droplets. Continuous condensation of vapor and glomeration of the small droplets will produce larger water droplets. Meanwhile, when the temperature of the cold surface is lower than the freezing point of water, the water droplets formed on the cold surface will freeze. Subsequently, frost will deposit on these frozen droplets. Frost deposition on heat transfer surfaces will produce a series of negative effects on heat transfer equipment. For example, heat transfer resistance and pressure loss will increase, resulting in poor heat transfer performance. In addition, the air passage may be blocked and mal-