The stress relaxation experiments of rocks under the deformation at peak load and load-unload experiments before peak load are carried out. The results of experiments show that the form of the relaxation curves of rocks under the deformation at peak load descends in step form; the stress relaxation of rock under the deformation at peak load is discontinuous

that is

alternant between quickly and slowly

which is the resut of the synthetic action of slip and extension of existing cracks and creation of new cracks; and integrality of rock structure and the performance of store of elastic deformation energy of rock have obvious influence on the number of stress descending steps and stress relaxation time.