Abstract: The thixotropy of a Bingham plastic fluid and the law of drag force flowing around a sphere in the fluid are studied in this paper. Through theoretical derivation and experiments,the formulas calculating the yield stress of Bingham fluids with thixotropy and the non-settling maximum sphere diameter of increasing with the duration of the fluid at rest were obtained. The Navier-Stokes equations of Bingham fluids with thixotropy were developed. By solving these equations,the formulas of the drag force and drag coefficient flowing aroung a sphere in Bingham fluids with thixotropy were derived also. Using data of rheological experiments with various slurries and of a sphere motion observed on X-ray screen,the above equations were verified with the results that the calculated values are in good agreement with the experimental values.