The flow through a rectangular curved duct is investigated numerically with emphasis on the secondary flow patterns. The non-dimensional curvature of the duct is taken to be 0.02 and the aspect ratios of the duct cross-section are considered to be 0.5, 2, 3 and 4. The calculation covers a wide range of the Dean number. The steady solutions are first calculated and then the linear stability of the solutions is examined. The multiple solutions and the bifurcation of the solutions are obtained. The critical Dean numbers at which the transition of the secondary flow patterns occurs are also obtained.