Abstract: This paper illustrates the fact that the actual shear strength of sound rock can be determined from the simple compression and tension tests of the rock sample in laboratory and is much larger than the result obtained from usual field shear tests. It is even more larger than the usual adopted values by designers. The reason of this phenomenon is discussed briefly and the use of this actural shear strength in the practical design work especially in the stability analysis of surrounding rock for underground constructions is presented.