This study presents the development and performance measurement of a 2.5 kW class free-piston Stirling engine with detailed design and fabrication processes, considering flexure spring stiffness effects on the engine dynamics and performance. The development of the test engine includes tube type heat exchangers and planar flexure springs. In particular, the flexure springs are designed via finite element analyses and verified through stiffness measurements and fatigue life tests. For the performance measurements, an engine test rig is constructed with an LNG combustion heat supply. The thermal efficiency is evaluated in terms of combustion gas components based on the reaction formula of methane. As a result, the electric output and efficiency of the test engine were measured to be 2.46 kW and 19.46%, respectively. Furthermore, the effects of displacer spring stiffness are experimented with a stack design of flexure springs by varying the number of springs. As a result, increases in displacer stiffness increased engine operating frequency and decreased both the piston amplitudes, while power piston velocity remained relatively constant as well as the engine power output. The experimental results are analyzed through a simple linear dynamic model, which showed reasonable agreements.