The shear strength on the slip surface of a slowly moving landslide is at residual value and the mobilized viscous component of strength maintains slide equilibrium. Object of the study is the evaluation of viscous component of residual shear strength of soil. Experimental analyses were executed by reversal direct shear tests on reconstituted samples. Available measurement systems were modified and a new procedure to determine the relationship between shear strength and velocity was developed. At residual strength the machine motor was stopped and increment of shear displacement associated to decrement of force is recorded. The relationship between shear force and velocity, in the range 10-1÷104 mm/year, was obtained.