This study measured the frost thickness and frost mass on a flat plate to propose the correlation equations for the local and average frost thickness, frost density, and frost mass. Key parameters were the cooling surface temperature of the flat plate from 258.2 to 268.2 K, absolute humidity of air from 2.98 to 4.16 g/kgDA, air temperature from 273.5 to 280.2 K, and air velocity from 1.0 to 2.5 m/s. A 50% ethylene glycol aqueous solution was used as a coolant. The sensitivity analysis of the parameters such as air temperature, air humidity, air velocity, and surface temperature on the frost thickness and frost mass were experimentally investigated under the heat pump condition. Correlation equations for the local and average frost thickness and frost mass under the heat pump condition were proposed. The values predicted by the correlation equations under the freezer condition were larger by a maximum of 30-50% than the values predicted by the present correlation equations under the heat pump condition. The proposed correlation equations might be applied to the part of the freezer condition.