The characteristics of combustion flame for premixed propane and air flowing inside an adiabatic refractory tube without a flameholder were numerically investigated. The effect of thermal conductivity of a tube wall k(w) and a tube aspect ratio L = l/D on the flame front was studied for the stable combustion at multiple locations. It is revealed that both upstream and downstream flame locations are shifted into the center of the tube with decreasing the thermal conductivity of the tube wall. Then the extinguishment occurs in the center of the tube because of insufficient thermal feedback. As the tube aspect ratio is decreased by the increase of tube diameter, either locus of the flame front moves to an inlet. On the contrary, when the tube aspect ratio is increased by the increase of tube length, the upstream flame location is almost the same place. The downstream flame location, however, moves to an exit. The location of stable flame appears to be determined by the distance to the nearest openings to the outside due to the thermal energy balance. The results are quite different from those for conventional combustion.