In refrigeration and air conditioning industries, to enhance heat transfer particularly in the evaporators, fin-and-tube heat exchangers are widely used. Due to low temperatures in such systems generally frost formation on such heat exchangers takes places. Frost formation and growth over any surface will influence the system operation and performance and it reduces heat transfer between the heat exchanger surface and ambient environment. Design of such systems requires the detail understanding of the frost nucleation, growth and shape with respect to time on various position of any surface such as plate fin and tube heat exchangers. Frost formation is studied extensively but from the review of the literature, it is observed that few reports about frost formation on plate fin and tube heat exchanger by natural convection are available. In this study attempt is made to investigate experimentally the performance of a plate fin and tube heat exchanger for variant refrigerant temperature and ambient conditions. Experiments are carried out in a special constructed control room. For experiments the refrigerant mean temperatures varies between -10 °C and -20 °C, ambient temperatures between 20 °C and 30 °C and relative humidity varied between 50 % and 70 %. In these experiments, variation of each parameter is studied while the others are kept constant. The results include changes of frost thickness and its impact on heat transfer rate under different environmental conditions. Frost thickness varies due to changes of velocity, temperature and concentration boundary layers over the heat exchanger and also with respect to time. By increasing of relative humidity and decreasing of refrigerant temperature, the thickness of frost increases but by increasing of ambient temperature up to 30 degree centigrade, frost thickness increases, but for higher ambient temperatures, frost thickness is not increased considerable. Also frost thickness at higher position of the fins is more than lower position. The rate of heat transfer initially increases and then decreases until reaches to a nearly a constant value. By increasing of each parameter of the ambient conditions, the rate of heat transfer increases. For practical applications dimensionless empirical correlations are developed for the frost thickness and the rate of heat transfer according to experimental data. A modeling of mass transfer is done by use of heat and mass analogy, assuming supersaturation condition on frost layer and good agreement is observed between the results of modeling and experimental measurement. Key words: Natural convection, Frost formation, Fin and tube heat exchanger, Environmental parameters.