Reusable rockets is emerging as a worldwide concern to drastically reduce operating costs in the development of space propulsion systems, and research on eco-friendly propellants is also being actively conducted. In particular, liquid methane is superior in terms of economy and performance compared to liquid hydrogen and kerosene, which are currently used as propellant for space launch vehicles. A rocket engine consists of a propellant system, an injector, a combustion chamber, and a supersonic nozzle, among which the injector is responsible for transforming the liquid propellant into dense droplets. An effective atomization of liquid propellants produced by the injector has a significant impact on the performance of combustion engines and thus the design optimization of injector is essential to enhancing the characteristics related to combustion efficiency and stability. Hence, as a step for the research and development of a small rocket engine using methane as fuel, the study was conducted as follows; an injector performance evaluation through cold-flow test and hot-firing test. In addition, as part of the propellant supply system optimization process, the cavitation venturi was designed and manufactured to determine the performance of the venturi. Among the diverse injectors used in liquid rocket engines, swirl-type injector has the advantage of exceptional spraying and mixing coming specifically from the tangential momentum of liquid flow. Performance of the swirl injector is primarily governed by its swirl strength, which is proportional to tangential momentum and inversely proportional to axial momentum. The swirl strength and the spray performance of swirl injector are affected by the design parameters such as the diameter of swirl chamber, the number of tangential inlets, and the recess ratio of injector. Therefore, the spray performance according to the design parameters of the swirl-coaxial injector and liquid-gas propellant mixing performance were analyzed in detail using a backlight system and a laser system. However, there is a limit to the test to observe the spray characteristics according to the atomization and mixing performance through the cold-flow test conducted at ambient pressure. Accordingly, hot-firing tests were performed to observe the effect of design parameters. After that, the combustion performance was calculated using NASA''s CEA code.