Effect of adding an inorganic salt, lithium chloride, and water on viscosity of an ionic liquid, 1-n-butyl-3-methylimidazolium chloride (BmimCl), was investigated by shear stress measurements with a rheometer. Shear rate dependence of viscosity showed shear thinning behaviour, which implies that some structure should exist in the liquid and the structure should change at high shear rates. Addition of LiCl enhances the viscosity of BmimCl. The logarithmic value of zero shear viscosity, η0, of BmimCl increases linearly and largely with increasing the added salt contents. The increasing rate of the viscosity by addition of LiCl was about ten times larger than that for aqueous solution of LiCl. When water is added into BmimCl, viscosity decreased. The increasing rate of the viscosity by addition of LiCl for BmimCl with about 5 wt% of water was almost the same as that for BmimCl without addition of water.