An electric power system comprising a two-stroke gasoline engine integrated with a generator and a battery is designed, and its performance is evaluated on the ground if it can provide constant 6 kW-scale electric power for UAVs. The reciprocal engine is fabricated to have opposed pistons connected to the crankshaft, which can provide the house to hold the generator at its end, resulting in a compact design. This system integration allows the generated power to be readily manipulated by adjusting the engine rpm. In addition, the generator operates in two modes: a starter mode when starting an engine initially, and then a power unit mode after reaching 4500 engine rpm. In turn, a power supply and starter mode were combined in a single unit. The battery is adopted to momentarily cover insufficient power when a high load is inevitable, and its capacity is tested by applying an 11 kW electric load that the engine-generator itself cannot cover. Thus, a hybrid electric power plant is formed by incorporating different power sources that can satisfy various load requests up to 11 kW, and it can extend flight endurance by alternating the main power between an engine generator and a battery.