Abstract: Why there would always be inconstancy between the loss of hydrocarbon generation potential and the D-values derived from the total hydrocarbon generation and expulsion from coal? A series of simulation experiments on secondary hydrocarbon generation from the Carboniferous-Permian coals may give the answer.It is probably caused by the great proportion of unstable non-hydrocarbons and asphaltine in degradable substances,the considerable part of which was transformed into insoluble organic matters or other non-hydrocarbons during evolution,resulting in D-values between expelled hydrocarbons and loss in hydrocarbon generation potential.Thus the current conception to account the loss in hydrocarbon generation potential as the value of hydrocarbon generation and expulsion should be corrected,while making resources evaluation.