This paper reports the experimental results of the basic performance of a GCH4/GO2 small thruster with a swirl-coaxial injector. The effects of the equivalence ratio of CH4/O2 mixtures and the swirl intensity of the O2 injector on the combustion characteristics of the combustion chamber were studied. It was found that applying swirl to the O2 jet greatly improved the thruster performance by increasing the mixing rate between the propellants. The performance was also influenced by the equivalence ratio. Direct observation in the combustion chamber showed that when the swirl intensity of the O2 jet was small, the flame lifted at low equivalence ratios and the rim stabilized flame was formed at high equivalence ratios. For higher swirl intensity, only a lifted flame was developed over the range of the equivalence ratio tested. The tendency is explained by the ratio of the angular momentum of the O2 jet to the linear momentum of CH4 jet. The lifted flames had a favorable effect on the mixing between the propellants, resulting in the good thruster performance. Emission spectra from the flame in the combustion chamber were measured to make the combustion process clear. The measurement showed that the various reactions including the generation of OH, CH and C2 initially proceeded on the injector side. The formation order and intensity variation of the species were discussed.