Abstract: As welll known,the instability modes of rocks depend on quite a number of factors,such as rock type,fault gouge,temperature,confining pressure,strain rate and liquid media,etc.It is emphasized in the paper that all of them are associated with the plastic components contained in rocks to different extent.The plastic components of rocks are referred to those of minerals which can deform irreversibly under the given geological and physical environment.Triaxial compression tests of intact quartzite,granite,granodiorite,gabbro and basalt under room temperature and confining pressures up to 0.7 GPa indicate that the contents of plastic components in rocks have no obvious effect on the magnitudes of stress drop of sudden instability,but do influence the instability modes.The plastic components filled in the elastic framework of rock affect the elastic behavior of rock specimen on the whole in two aspects: (1)The effective bearing area of specimen and the effective height of reversible deformation part in specimen are reduced and higher stress level is necessary to accumulate enough reversible strain energy for sudden instability; (2) The plastic components,as damping elements,hinder the rebound of elastic framework and higher strain energy is necessary too.