Liner cavitation induced by piston slap in a diesel engine is caused by water pressure fluctuation when the pressure of coolant falls below saturated vapor pressure. Cavitation erosion of cylinder liners is thought to be generated by the impulsive pressure or jet flow impingement following the collapse of cavitation bubbles. In this study, a numerical method to predict the water pressure fluctuation in water coolant passage induced by piston slap impact force is developed. In complimentary impact vibration experiments, high frequency components of the water pressure fluctuation can be seen just after the pressure reaches the saturated vapor pressure level or less. These high frequency components seem to show the occurrence of cavitation. A finite element acoustic model of the water coolant passage in an actual engine block is created and its validity is confirmed by the acoustic vibration tests in air. Then, the coupled vibration characteristics of the water acoustic field and engine block structure are determined, and water pressure waveform induced by piston slap is predicted.