Abstract: The pressure difference between initial coal seam pressure and critical desorption pressure was neglected in previous material balance methods. Therefore, a modified method has been presented in this paper. In the derivation, the initial gas content is expressed as a Langmuir's equation about the critical desorption pressure, rather than the initial coal seam pressure. Finally, a linear material balance equation of the pseudo average reservoir pressure (P/Z^*) versus cumulative gas production (G_p) is obtained, whose intercept on horizontal axis yields the initial gas reserves, and intercept on vertical axis gives the pseudo critical desorption pressure, instead of the pseudo initial reservoir pressure. By using this method to evaluate a simulated reservoir, its reserves can be obtained with an error less than 0.35%, which indicates the method is reliable when the Langmuir's volume and Langmuir's pressure, average reservoir pressure, initial cleat porosity are accurate.