In this paper the experiments of hydraulic free piston diesel engine is described. The experimental data were obtained from measurement instruments on the free piston diesel engine that has been developed by Beijing Institute of Technology [ 1 ]. This article discusses the influences of compression pressure, injection timing, and combustion process to the free piston diesel engine principle. The compression process experiment shows that the piston velocity, the compression ratio can be controlled by adjusting the compression pressure. With the increasing of the compression pressure, there is a growing a compression ratio and piston velocity. The study on injection timing shows that the injection timing impacts the cylinder pressure peak value and the pressure peak arrival time. The combustion process is quite different from the crankshaft engine because of the unique piston movement characteristics of the hydraulic free piston engine.The optimum ignition delay point of the hydraulic free piston diesel engine is 24mm before top dead center, the compression pressure is 18MPa. The analysis on the hydraulic free piston diesel engine experimental results showed the key impact factors of high frequency operation of the proposed diesel engine. It provides a basis for the next step to implement high frequency operation.