Abstract: Some of the existing theories of the shear strength for cohesive soils, including the Theories of Krey-Tiedemann, Hvorslev, Ohde, , Haefeli, Skempton-Bjerrum, Skempton-Bishop and Rendulic-Henkel, were critically reviewed. Based on these observations, an unified comprehensive theory of shear strength for normally consolidated saturated clay was proposed to cover all kinds of test results of shear strengths in terms of total stress, effective stress and true shear strength parameters, and to establish the relationship between them. In this theory there are seven shear strength curves as follows:' line-Shear strength curve in terms of effective stress determined fromconsolidted-undrained triaxial compression test with measurement of pore pressure, .d line-Shear strength curve in terms of effective stress determined fromdrained triaxial compression test, .e line-True shear strength curve, where pc is the equivalentconsolidation pressure on the primary consolidation curve corresponding to the water content of soil specimen at failure.u = 0 line-Undrained shear strength curve under loading condition,cu line-Consolidated-undrained shear strength curve indicating the variationof strength with normal total stress on the failure plane, .cq line-Consolidated-undrained shear strength curve indicating the variation of strength with consolidation pressure before shearing, .r line-Consolidated-undrained shear strength curve under unloading condition, , where pe is the equivalent consolidation pressure on the primary consolidation curve corresponding to the water content of soil specimen after unloading and swelling or reloading and consolidation.There are altogether 8 parameters （', d, c, ξ, u = 0, cn, cq, and r） above. But actually, there are only 2 basic parameters: ' and c. All other parameters are merely functions of these basic parameters. For instanceCO Sc Hence the following expression can be obtained:This expression can be used to determine the true angle of internal friction e of normally consolidated saturated clay from the values of ', cu and Af obtained with conventional triaxial tests.