The second-law analysis of the combustion of methane with oxygen or air under different inlet reactant temperatures (300-600 K), ambient pressures (1-75 bar) and mixture strength (0. 1-1) is performed. The results of second-law effectiveness are presented in such a way as to bring out clearly the effects of the different parameters. In general, conditions promoting high flame temperature and large number of product moles result in high values of effectiveness. The results show that the effectiveness increases with increasing inlet temperature and increasing ambient pressure, the effect of inlet temperatue is, however, more pronounced than that of ambient pressure.