This paper reports an experimental research in suspension rheology in which it was found that the yield stress τ0 is controlled by the shear stress rate in the deformation process, conjunctioned with the relaxation time tR coming from the suspension memory. Other parameter that affects the Yield Stress τ0 is the particle packing tested by means of the volumetric solid fraction and the particles size distribution in the suspension. The main conclusion is: Yield Stress is not an invariant property of the suspension; on the contrary, there exist several Yield Stress values depending on the dynamic deformation There is even a particular case where the Yield Stress value may be cero.