Abstract: Triaxial compression with 3=1.3kb is applied to samples of Jinan gabbro and the development of microcracks is studied with a microscope. It is found that under low stresses, the microcracks occur within the crystal grains of the rock minerals and their orientation is determined principally by the configuration of the crystals. When the applied stress reaches about 80% of the ultimate strength, the cracks become more concentrated in the central part near one diagonal of the sample. At this time, there appear a few microfalts, which cut through several crystal grains. The orientation of these microfaults depends obviously on the direction of the external stress, which makes an angle less than 40 with the direction of maximum stress1. The volume changes as wellaas VP and Vs in the rock samples are also measured. It is found that the development of the microcracks in rock samples and their influence on physical propertied of the rock are dependent on the dimensions of the crystal grains.