The paper presents the results of an experimental study of the fluid viscosity on the clay behavior. The clay behavior, which was studied in this paper are the clay consistency, the hydraulic conductivity of the tested clay, the clay compaction, and the clay consolidation. The clay consistency, which means the clay ability for deformation, may be in liquid state, plastic state, semi – solid state or solid state according to limits which are called Atterberg limits. These limits which include liquid, plastic and shrinkage limits depend on fluid viscosity. In this study, a laboratory tests were carried out to show the effect of the fluid viscosity on these limits. Also, four different fluids were used to study the effect of fluid viscosity on the clay compaction and the optimum fluid content and the corresponding maximum dry density of the tested specimen. These fluids were used to study the effect of the change of fluid on the clay hydraulic conductivity using the falling head test. It is found also that fluid viscosity has a direct effect on the clay consolidation. Results are given by curves to show the effect of fluid viscosity on clay hydraulic conductivity, compaction and consolidation