The paper presents a brief description of a new method of measurement of the heat of combustion of gases and the design of a new gas calorimeter. The results of measurements of the thermal power and the heat of combustion of methane are analyzed with the aim to determine their uncertainties. It is proposed to divide the sources of uncertainty in three groups depending on the method of their processing. The investigations carried out prove the correctness of the measurement of both the thermal power and the heat of combustion of methane. The following values of the uncertainties of the heat of combustion of methane were determined: the relative standard uncertainty Type A less than or equal to0.02% and the combined relative standard uncertainty less than or equal to0.04%. The obtained results permit recommending the proposed gas calorimeter as a prototype of a standard gas calorimeter.