The experimental study on sudden instability processes of the intact Juyongguan granite and Qian' an quartzite specimens and those of Juyongguan granite with saw cuts are carried out at room temperature and solid confining pressures, using the high speed data collecting system with a video-tape recorder. The result indicates that when specimen slides rapidly along its internal oblique fracturing or sliding belt and instability takes place suddenly in the system, the stresses and displacements of different measured points in the testing system vibrate, attenuate, and tend to a new stable terminal state. The whole dynamic process can be divided, in terms of initial motion semi-period of the vibration, into two stages, frictional sliding stage and free vibration one. The synthetic dynamic stress-strain (relative displacement) curves are created from the stress and displacement data recorded. It is also shown that the maximum dynamic sliding displacement is equal or approximate to that of terminal state and the ratio of dynamic stress drop to terminal one is about 1.6.