Abstract: When in Institute of Soil and Rock Mechanics, Karlsruhe University, Federal Germany, the author of this paper performed some cyclic loading tests on the large-sized triaxial testing machine developed in this institute. The rock sample used were 60cm in diameter and 120cm in height. The work mainly included the study of deformation and strength behavior of the samples and was based on the viewpoint of analyzing the irreversible deformation. Under cyclic loading, although the irreversible deformation generally increased with the n umber of cycles, the deformation trends obtained at various stress levels distinctly differed from each other. At low stress levels a comparatively large irreversible deformation arouse from the first few cycles of loading. With the increase of the number of cycles its growth decayed gradually and tended finally to a stable constant value. When the stress level reached a certain “threshold” value, the situation became entirely different. The increase of the number of cycles caused an acclecrated growth of the irreversible deformation till a complete failure of the whole sample. The test results also illustrated that this characteristic stress value was slightly lower than the so-called “yield” value. The study of such a characteristic value is of great importance for under-standing the mechanical properties of rock masses under cyclic loading. The experimental curves of irreversible deformation vs. the number of cycles show a remarkable resemblance to corresponding rheological curves, which, perhaps, may not be accidental. It is propose in this paper that the relation of the strength obtained in conventional tests, the long-term strength in rheological tests and the strength under cyclic loading should be studied. Undoubtedly, it is significant for evaluating the strength and safety factor in rock engineering problems.