The present study has set forth a hypothesis of impermeability characteristics of caking coals. The impermeability of coal in plastic state to gas is an extremely important property of caking coals. The formation of impermeable plastic mass is a necessary condition for the caking and coking process. The impermeability creates a cage effect for physiochemical surface process which is caused by and promotes complicated chemical interractions between pyrolysis products. The determination of impermeability in plastic state of coal along with other related parameters should be beneficial for making an overall evaluation of caking behavior. Based on the penetrative plastometer, a modified experimental instrument was designed and constructed. The unique advantage of measuring impermeability of coal in its plastic state by this instrument is providing a means to synthesize the impermeability with other data such as volumetric shrinkage and coke quality. The parameters for identifying impermeability of various coals have been recognized as the maximum value of resistance, the initial temperature of intense rising resistance and the temperature of maximum resistance. The impermeability of coals in plastic state depends on their original properties. Nevertheless, experimental results had shown that the impermeability could be improved by way of regulating processing conditions. This in turn may be helpful to broaden the marketability of otherwise inferior caking coals.