Abstract: Flow characteristics and local losses in transition section from trapezoidal open channel to horseshoe-shaped tunnel has been studied and tested on a hydraulic scale model of the western main canal of water diversion project in Karamay. The model made of plexiglass, had a geometric scale of 1/25 in accord with Froude similitude. Longitudinal water surface profiles were measured in steady flow conditions of flow rate range of 3.88 to 17.05 L/s. At a flow rate greater than 13.37 L/s, water level was rising rapidly near the tunnel entrance and fluctuating in the tunnel. This should be a phenomenon caused by the flow turbulences generated by wall contraction. Minor loss coefficients of the transition section in a range of 0.19 to 1.24 were calculated using the measured water surface profiles and cross-sectional average velocities. As the contraction angle of the transition increases, the minor losses were decreasing first and then increasing. To solve the problems with the original design of the tunnel found in our experiment, the transition section was lengthened and the lengths of warped surface channel and rectangular channel were optimized. Measurements show smooth streamlines and a reduction in water surface fluctuations in the modified transition, along with a reduction in its minor loss to a range of 0.05 to 0.82. The results are helpful to the design of transition section from trapezoidal open channel to horseshoe-shaped tunnel in real water diversion projects.