Abstract: Thermal efficiency of heater is generally calculated by the positive balance method, which involves direct measurement of input and output of the heater. To determine correlation between the thermal efficiency and such controllable parameters as gas quality, exhaust temperatures, and excessive air coefficient, the thermal efficiency calculation method is modified, namely, the negative balance method is used to calculate thermal efficiency. This method can determine the thermal efficiency through calculation of exhaust losses, heat losses and incomplete combustion of gases. In accordance with the thermal calculation method, calculation software is programmed. In addition, process parameters are maintained in the software calculation interface. Thus, basic data can be provided for identification of factors that may affect thermal efficiency, stipulation of energy-conservation measures and assessment of energy-conservation measures. The proposed calculation method and software have been applied in some oilfield to determine thermal efficiencies of heaters of different models.