Summary: The steady helical flow of an incompressible power-law fluid in the region between infinitely long coaxial circular cylinders is analyzed, in the case when the intercylindrical gap is small in an appropriate sense. A perturbation method based on a scaled form of intercylindrical gap size as perturbation parameter is applied to give leading-order expressions for the components of velocity field, as well as for other quantities related to the flow. The results of this analysis are compared with those obtained from a full numerical solution of flow equations for fluids of practical interest.