Using methane as the carbon source to lower the coke reactivity and enhance its strength is a promising method. However, there is little literature on the thermal performance of metallurgical coke under methane. In this work, thermal performances of metallurgical coke and the gas evolution under methane were studied by TG-MS to well understand the interaction between methane and metallurgical coke. Effects of experimental conditions including temperature, holding time, partial pressure and flow rate of methane, and the quality of cokes on the weight gain of cokes were investigated. The gas evolution including CO2, H2O and H2 during the process was examined by MS. The results show that only weight loss is found when the metallurgical coke is heated under inert gas while the weight gain begains at 860 ℃ when it is heated under methane. It is found that no weight gain and methane decomposition reaction occur in the absence of metallurgical coke in the reactor. It indicates that the metallurgical coke has some catalytic effects on the decomposition of methane. The results of MS analysis show that the weight loss at the initial stage is mainly caused by the evolution of CO2 and H2O, while the weight gain of coke above 800 ℃ is due to the carbon formation on the coke by the CH4 decomposition to release H2. At the constant temperature, the rate of H2 evolution does not change greatly in the holding time. The experimental conditions and the quality of the coke also have great effects on the methane decomposition. The weight of cokes increases with increasing temperature, holding time and partial pressure of methane. Flow rates of methane in the experiment have little effect on the weight gain of cokes. The poorer the quality of metallurgical coke is, the higher the increase of the coke weight will be, which shows that the thermal processing in methane could improve the quality of metallurgical coke.