This paper examines the critical air temperature at which frost formation does not occur on a cold plate in order to improve the efficiency of a heat pump. The critical air temperature on test points is obtained from numerical analysis and a correlation of the critical air temperature is derived as a function of frosting factors (air absolute humidity, air velocity, and cold plate temperature) by least square method. Experiments are carried out to verify the correlation proposed in this study, and the prediction obtained with the correlation agrees well with the experimental data. Based on the correlation, we propose the non-frosting regions according to frosting factors.