Although the pyrolysis of coal has been studied for centuries, only few methods, which can evaluate the amount of volatile matter in the case of a change in the reaction mechanism with pyrolysis time, have been developed. In this study, a method for accurately predicting the volatile amount of brown coal depending on temperature and time of pyrolysis has been studied in isothermal condition. This method is based on the kinetic parameters obtained from the pyrolysis process. The kinetic parameters were determined using the first-order series reaction model, which consists of two steps: the first step is rapid weight loss with free radical reaction, and the second step consists of slow weight loss with condensation reaction (char formation). Both steps follow a first-order reaction with different reaction rate constants, which are obtained from the graph of ln(1 - X-A) versus the pyrolysis time at each given pyrolysis temperature. The validity of this new method for estimation of the final amount of volatile matter was proven by comparing these results with the experimental results of the pyrolysis of three different Mongolian brown coals. In this estimation, the average absolute error (AAE) is very low (<2%) and the correlation coefficient (R-2) is very high (>= 0.99). This proposed two-step reaction model gives a good prediction of the final volatile amount, and the calculated results are in good compliance with the experimental values.