As a hot-firing performance evaluation for the design optimization of GCH4-LOx small rocket engine employing film-cooling system using LOx as a coolant, the following research was conducted; a characteristic length optimization for the combustion chamber of GCH4-LOx small rocket engine and performance evaluations varying propellant supply-conditions of small rocket engine with film-cooling applied. Attention has been received to the necessity for reusable launch vehicles with the intention of reduce the dramatic cost of development for space propulsion engine, and a need for research of eco-friendly propellants has also increased. Among various fuels, liquid methane is considered appropriate as a high-performance/low-cost propellant because it is superior in terms of performance, eco-friendly and economy compared to liquid hydrogen and kerosene. And application of a cooling system is essential to rocket engines because of an extreme heat flux coming from hot combustion gas. In general, regenerative-cooling and film-cooling are used to protect combustor and nozzle from the thermomechanical damage. Regenerative-cooling has the advantage of minimizing heat loss and improving combustion efficiency. However, this method has a disadvantage of system complexity which results in poor reliability as well as manufacturing difficulty specifically related with geometric limitations in small engines. Contrariwise, film-cooling is more effective than the former in small rocket engines which needs the less area of cooling, such as vernier engine and orbital maneuvering thruster, because of the system simplicity. Besides, when the film-cooling is applied to staged combustion-cycle engines, a preburner operating in a fuel-rich/-lean condition can supply a high enthalpy propellant to the main combustion chamber without wasting the film-coolant of preburner. Additionally, the film-cooling can be employed not only for an upper stage engine but also for an 1st-stage engine of launch vehicle when combined with regenerative-cooling system. Inasmuch as these application points, it can be said that the film-coolings are taken up on various scale of rocket engines. The optimization of the chamber characteristic length was performed for that is one of the critical design parameters of a small rocket engine. Performance characteristics such as thrust, characteristic velocity, and specific impulse at the steady-state could be acquired as the crucial parameters of the engine performance. The performance characteristics obtained through the test were evaluated and analyzed make the use of the theoretical performance which is calculated from CEA (Chemical Equilibrium with Application) code. And in order to derive the optimal operating-condition of the GCH4-LOx small rocket engine employing film-cooling system, hot-firing tests were conducted under various propellant supply-conditions. The operating-condition optimization study was analyzed through the trade-off between combustion performance and cooling performance attributable to the propellant supply-conditions including coolant.