The flow structures in bends of narrow open channels are considered to be affected by the configuration of cross section. This study aims to clarify the secondary flow structures in curved open channels withtrapezoidal cross section experimentally, for three types of side slope. The obtained structures of secondary flow, primary velocity and bed shear stress were almost similar to those in the rectangular curved open channels. However, a counterrotating vortex near the outer bank was diminished with a decrease of the side slope. The developing process of the secondary flow strength was not so much changed from the rectangular case. The structure of the primary mean velocity and the bed shear stress were distorted according to the structural change of secondary flow.