Abstract: On the basis of principle prototype of a single piston hydraulic free-piston diesel engine (HFPDE), the cycle-to-cycle variations in cold starting process were investigated. The results indicate that the cold starting process was completed after 150 engine cycles. During the cold starting process, a few of dependent relationship could be observed as follows. The variation of bottom dead center reduces as engine cycles increase, so does piston bouncing and dwelling position. The maximum pressure in cylinder is reduced gradually. The peak pressure rise rate is larger than critical value and fluctuates greatly until stable operation. The peak heat release rate also decreases with the increase of engine cycles. As engine cycles increase, the time duration at pre-mixed combustion phase is almost constant, while it becomes longer at mixing-controlled combustion phase and late combustion phase. The combustion process gradually stabilizes after cold starting process.