Abstract: fn this paper, presented are the results of experimental studies during the past seventy years on the index properties of cohesive soils at two consistency limits, that is, the liquid and plastic limits. Based on the correlation that the undraincd shear strength of remolded cohesive soils depends solely on their water content, the strength properties at liquid limit and at plastic limit have been evaluated. Finally, the reversal extrusion method, as a new method for determining plastic limit according to the corresponding shear strength characteristics are described and considered to be promising future development. This method is simple, economic, reliable and acceptable in accordance with the principles of mechanics and plasticity.