The inertia of the fluid and the inner cylinder in co-axial cylinders rheometer have a great influence on the unsteady flow of non-Newtonian fluid. In the paper this phenomenon is studied minitely and a method to correct the measured results for an unsteady flow in the rheometer is proposed. It was found that the inertia effect of a fluid can be ignored when the gap between cylinders is smaller and the inertia effect of the inner cylinder can be represented with a vibration equation. Using the equation changes of Huang's blood constitutive parameters are observed.