As a first step to establish the law governing shear failure of typical crustal materials in the brittle-plastic transition regime under lithospheric conditions, and thereby to properly estimate a depth profile of lithospheric strength in quantitative terms, the effects of the normal stress &sgr;n across the fault surfaces and ambient temperature T on the shear failure strength of dry Westerly granite in the brittle to brittle-plastic transition regimes are evaluated quantitatively, using experimental data published by earlier authors. The empirical law proposed can predict the shear failure strength at strain rates of 10-4--10-5/s under any (&sgr;n, T) conditions in the brittle to brittle-plastic transition regimes. ¿ American Geophysical Union 1995