Secondary flow plays an important role in a number of areas in hydraulics, such as sediment transport, distribution of primary flow velocity and boundary shear, flow resistance, and stream meandering. In this study, secondary currents generated in a straight, triangular open channel are investigated and used to develop a method of calculating secondary flow in an entire transverse cross section of a straight channel. The method developed involves the use in different regions of a cross section of different sets of curvilinear coordinate systems consisting of isovels of primary flow and their orthogonal trajectories. Descriptions of the method as well as calculated results are presented.