The moisture absorption experiments of two kinds of molding compound are conducted. The diffusion at low temperature and humidity observes the Fick’s Law, but it does not strictly obey at higher temperature and humidity. The phenomena can be attributed to the occurrence of the second phase, and the Fick’s Law can be modified by diffusion coefficient varying with the moisture concentration. The predicted value by the modified Fick’s Law agrees with the test data. At certain temperature, the saturated moisture content of molding compound is proportional to the relative humidity, and the proportional factor is the product of solubility and saturated vapor pressure. The solubility is a physical characteristic, which has no relations with the relative humidity of ambient air.