The Secondary flow in a curved duct of square cross-section, is investigated numerically using the spectral method. The walls of the duct except the outer wall rotate around the centre of curvature and an azimuthal pressure gradient is imposed. Multiple solutions are obtained at the same flow condition. It is also found that the secondary flow in a cross-section consists of two-vortex, four-vortex, eight-vortex or even non-symmetric vortex with respect to the centre line of the cross-section. The linear stability analysis for the multiple solution is also carried out to investigate the actual flow patterns.