Abstarct: In canal applications where diversion is required but the freeboard, the distance between the water level and where flooding would occur, is limited, a normal linear weir may not have the capacity to pass the required flood discharge with the limited freeboard available. Increasing weir length is one way to increase discharge capacity without raising the total upstream head. In this thesis flow over oblique weirs in one rectangular channel are studied and three-dimensional numerical modeling has been done by FLOW-3D software has been done. Then according to study of other researchers and published experimental results, validity of numerical model is evaluated. After validation of numerical model and compering numerical results with physical model results, it was observed that present model is acceptable and with changing various parameters in three dimensional model can examine the effect of input flow, height of weirs and angel of the weirs of discharge coefficients.