Abstract The Casson rheological model gives a better description of the rheological characteristics of drilling fluids than the Bingham and power law models. In this paper, a method for experimental verification is given, relevant test results are reported, flow profiles in pipe and annulus are analysed and several rheological formula concerning pressure drop, thinning index are derived. Introduction Drilling muds are fluids with complicated constitutions. Both the Bingham and power law models have been widely used in the research for the description of the shear stress vs. shear rate relationship in drilling fluids. As it is well known, the Bingham model is good enough in medium and high rate ranges.