Abstract: The condensation of the water vapor on the product surface in the thermal cycling test may cause irreversible damages in the electrical products. In this paper, the condensation is observed on the test sample with a large thermal inertia in a typical thermal cycling test chamber, and the source of the moisture is analyzed. It is shown that the pressure in the chamber drops with the decrease of the air temperature. The negative pressure at the fan inlet results in the infiltrating of the outside moist air into the test chamber. In addition, the frost layer on the evaporator in the air handling unit before the test is also an essential factor that levels up the dew point of the air in the test chamber during the temperature rise period. The nitrogen flow control can help maintain a positive pressure within the test chamber and effectively bring down the dew point temperature during the temperature rise period.