In order to understand the combustion stability of a methane (CH4)/oxygen (O-2) bipropellant as a next generation rocket liquid propellant, the combustion stability limits and morphology of gaseous CH4/O-2 (GCH(4)/GO(2)) coaxial jet flames, among various phases, in a single-element combustor are experimentally studied compared with the gaseous hydrogen/O-2 (GH(2)/GO(2)) coaxial jet flames. Only the stably attached flame and blowoff regimes are observed for both the GCH(4)/GO(2) and GH(2)/GO(2) flames, showing the flame thickness smaller than the injector lip thickness. Although the combustion stability limits of the GaH4/GO(2) flames are narrower than the GH(2)/GO(2) flames, practical use of CH4 in rocket engine applications seems to be acceptable since the fuel-rich CH4/O-2 flames show very stabilized and intensified burning. For the GCH(4)/GO(2) flames, the outer flame generated by the recirculating O-2 is relatively weak and OH* is distributed up to the downstream. With increasing O-2 injection velocity the length of the GCH(4)/GO(2) flames and the location at the maximum OH* intensity increase even under turbulent combustion condition, due to the saturated enhancement of CH4-O-2 diffusivity and the strong burning of pure O-2 near the injector lip. The present results provide a useful database to model combustion of CH4/O-2 bipropellants under various phases. (C) 2017 Elsevier Ltd. All rights reserved.