Abstract: An experimental study on variation of density and elastic wave velocity on nine typical rocks of North China was made by using a high presswre triaxial vessel. The maximum pressure in the experiments was 10.5kb. The volumetric strain was measured by means of resistance strain gauges and the variation of density was thus deduced. The elastic wave veloity was measured by the high frequency impulse method. Then the acoustic impedance under high pressure was calculated by parameters of density arid wave velocity. Prom the variation curves of volumetric strain and wave velocity with changes in pressure,the following quantities were calculated: the bulk modulus,the void ratio of cracks,the pressure of closing cracks,and the difference between the values of wave velocity above the pressure when it begins to change steadily and that at atmospheric pressure. The applications of these parameters to the earth sciences and explosion engineering was briefly discussed.