Cooling capacity and cool-down time of a Joule-Thomson (J-T) refrigerator depends on mass flow rates of refrigerants and their enthalpy changes. It is obvious that the mass flow rate through the J-T refrigerator depends on supply pressure; it increases as the supply pressure increases. Furthermore the mass flow rate increases as the temperature of the cold end decreases when the supply pressure does not change during the cool-down process. In this study, the cool-down characteristics of a miniature J-T refrigerator (fixed orifice / demand flow type) with different pressures of nitrogen and argon gas, were investigated by experiment. To analyze the cool-down characteristics of the refrigerator, the supply and exhaust pressure, the temperatures on the surface of the cryostat, and the mass flow rate were measured simultaneously. The results show the effect of the supply pressure on the performance of the refrigerator. The influence of the supply pressure and the temperature on the mass flow rate during the cool-down stage is discussed in detail.