Large eddy simulation of fully developed turbulent flow in a compound open channel was carried out using the Smagorinsky model. The Reynolds number based on hydraulic radius and cross sectional averaged velocity is 5300. The mechanism of secondary flow is discussed by evaluating terms in the momentum equations for mean secondary flow. To clarify the important role of pressure gradients in driving the secondary currents, pressure is decomposed into several components that account for the contributions of convection, external force, the Reynolds stresses, and the viscosity. It is found that not only the Reynolds stresses but also the pressure gradients induced by the normal and shear Reynolds stresses play a major role in generating secondary flow.