In terms of tensor analysis technique, the Navier-Stokes equations in helical coordinate system were derived. A steady incompressible flow of power-law fluid in helical pipes at low Reynolds number was investigated by the perturbation method. A second order solution of secondary flow was worked out. The secondary flow characteristics in helical pipes are analyzed. The effects of the number Dn, curvature and torsion on the secondary flow at different flow parameters were discussed. The results show that the secondary flow pattern changes from a single vortex to two vortices as the number Dn increases at a given curvature and a given torsion. Because of the effect of torsion, the secondary flow pattern changes from two almost symmetrical vortices to a single vortex as the torsion of the helical pipe increases while the Reynolds number and curvature hold constant. The secondary flow pattern cannot be affected by the curvature of the helical pipe at a given Dn number. A terms incompressible flow of power-law fluid in helical pipes at low Reynolds number was investigated by the perturbation method. A second order solution of secondary flow was The effects of the number Dn, curvature and torsion on the secondary flow at different flow parameters were discussed. The results show that the secondary flow pattern changes from a single vortex to two vortices as the number Dn increases at a given curvature and a given torsion. Because of the effect of torsion, the secondary flow pattern changes from two almost symmetrical vortices to a single vortex as the torsion of the helical pipe increases while the Reynolds number and pitch hold The secondary flow pattern can not be affected by the curvature of the helical pipe at a given Dn number.