Abstract: Rock is heterogeneous material with various flaws. The ideal strength of rock specimen can be determined from its complete stress-strain curve. After specimen is failed the experiment, reducing confining pressure and keeping the constant axial deformation, was carried out. According to the experiment the relationship of loading capacity of specimen, which is sustained by the internal friction, with confning pressure is given. To compare the relations of residual strength and peak strength with confining pressure,the internal friction coefficient in the yielding process is studied. The effect of internal friction coefficient on deformation and strength of various rock specimens is discussed. For siltsand, the internal friction coefficient reaches the maximum at the peak strength, and then the cohesion or material strength decreases only, so the confining pressure does not influence the specimen's deformation beyond the peak stress.For marble, the internal friction coefficient increases in the whole yielding, and the increasing of internal friction is various with the normal stress on the shear surface or confining pressure. At high confining pressure, the maxitmum axial stress is realized by internal friction when material strength of the specimen has wholly lost. The increasing of internal friction coefficient results in the transformation of brittle to ductile.