Gas-phase oxidative coupling of methane (OCM) was investigated with empty tubular-how reactors made of various materials (quartz, ceramic and stainless steel tubes). When the temperature is higher than 680 degrees C, the CH4 conversions in three kinds of empty reactors are measurable, and the gas-phase OCM becomes noticeable at 750 degrees C or higher. High temperature is beneficial to the activation of CH4 and the dehydrogenation of C2H6 to C2H4. Residence time of reactants in the heated volume plays an important role in the gas-phase OCM. Experiments indicated that the gas-phase OCM can be minimized by increasing reactant gas velocity or filling the free volume of the reactor with inert materials. Reactor surface also has an effect on the reaction and may participate in some surface reactions. Based on the kinetic results and the SEM investigation on the surfaces of reactors, a gas-phase reaction mechanism of OCM is proposed.