Viscosity of clay based suspensions is an important property in material science, chemical engineering and geology. Previous research on clay deposits in Latvia are based on three fields of usage - production of bricks and ceramics, sough and ceramsite. Properties for other application have been studied only in recent years, anyway there is a lack of information about viscosity and the factors affecting it. Viscosity of clay-based suspensions is based on interaction between clay particles, therefor causing a formation of agglomerates with various size. The stability of these agglomerates and also viscosity is influenced by the size and concentration of clay particles, presence of various compounds, pH values and applied force. Illite is the most abundant clay mineral in Latvia. The results shows, that with increasing the speed of rotation the viscosity decreases several times, indicating on pseidoplasticity. In suspensions greater than 30-35% of clay mass concentration the viscosity rapidly increases.