An important unresolved problem in geotechnical engineering practice is the determination of the shear strength of the soil obtained from a laboratory test for slope stability analysis. In certain cases, there are wide discrepancies between laboratory test results and the actual in-situ values of the shear strengths of soils. It is necessary, therefore, to develop laboratory procedures whereby a reliable result for engineering design can be obtained, and to determine shear strength values along slip surfaces relevant to different types of landslides. The emphasis of this study is to obtain relevant shear strength parameters for landslide clays from laboratory tests used in slope stability studies. This paper initially describes testing technique, preparation of sample and modification of the direct shear box apparatus during a series of tests. The shearing surfaces of landslide clays were then observed by using a scanning electron microscope to determine the influence of reversal shear on the clay particle reorientation and the difference in microstructure characteristics between the undisturbed samples and the remolded samples. The shear strengths from laboratory tests were compared with the computed values for typical landslides in Tertiary mudstone to confirm their reliability in slope stability analysis. The comparison of laboratory test results and computed results indicates that the residual strength parameters for the reversal direct shear box tests on the undisturbed samples should appropriately be used for evaluating slope stability, whereas the creep yield values agree with the computed ones for an initial slide in a non-fissured Tertiary mudstone.