A numerical solution is presented for the helical flow of a power-law fluid between coaxial cylinders. The solution is valid for isothermal steady creeping flow in an annular space of arbitrary thickness. Results are presented in the form of relations between dimensionless integral flow characteristics (e.g. pressure difference, volumetric flow rate and torque acting on the inner cylinder). Resulting graphs are valid for arbitrary radii ratio and have the flow behaviour index as a parameter. Modified rotational rheometer with coaxial cylinders was used for experimental verification of numerical solution. System of coaxial cylinders having the radii ratio 0.805 was used in experiments. Used model power-law fluids had the flow behaviour index in the range 0.35-1.0. The agreement between theoretical and experimental values of dimensionless pressure difference was found satisfactory. Experimental values of dimensionless torque were about 20% higher than values calculated for corresponding flowrate and rotational speed.