An investigation was performed to examine the initial temperature effect of gaseous reactants on the stability of a methane/oxygen diffusion flame for a constant oxidizer-to-fuel mass ratio (( O/F)(mass)) of 1.3 in a furnace. Gaseous reactants were individually heated and then ignited by an ethane/oxygen torch. Stability maps were developed based on data from over 130 combustion tests with initial reactant temperatures ranging from 298 to 398 K. As oxidizer initial temperature was increased, the Reynolds number and ignition time decreased, resulting in a quicker transition from a stable flame regime to an unstable flame regime. The diffusion flame standoff distance from the injector exit plane rose and fell as the oxidizer temperature was further increased. This indicates that for shorter ignition times and higher flow velocities associated with hotter reactants, diffusion flames achieved more efficient transverse mixing in the jet and thus minimized their instability associated with large standoff distances.