Abstract Spillway is an important hydraulic structure for the regulation of water level and the discharge of excess flood flows from the reservoir. The main aim of the present study was to numerically simulate flow over Ogee-type spillways, using the well-known Flow-3D model. Nazloo Dam Spillway is under construction on the Nazloo River (in west side of the Urmia Lake basin, Iran). Physical model of this spillway (with scale 1:40) was constructed and tested by Water Research Institute of Ministry of Energy, Iran. Flow pattern over the Nazloo spillway was simulated for three head water levels corresponding to low, mean and high flows (i.e. 497, 830 and 2272 m3/s, respectively). Three-dimensional geometry of the spillway was generated using AutoCAD software. The extent of numerical grids was tested with 5 different sizes from 0.6 to 6 cm. Three-dimensional grid of (183.3×175×53.3) with size of 0.6 cm was well adopted considering stability criteria. For inlet boundary condition, Specified Pressure option; for outlet boundary condition, outflow option; and for other boundaries, the symmetry boundary condition was introduced to the model. The simulation results were compared with Corresponding experimental results from the physical model. Sensitivity analysis indicated no significant dependency of the model to the turbulent mixing length and the surface roughness of the spillway. Three turbulence models (K-, RNG and LES) were examined, and the RNG model was better adopted. A total of 43 runs were carried out: 16 with no piers, 21 using one pier and 6 runs with 2 piers on the spillway crest. The computing time for simulating flow was 20 and 40 seconds for runs without and with piers, respectively. Comparison of the simulated results with the corresponding experimental data proved the capability of the model for reproducing the flow pattern over such high spillways. Keywords: Ogee spillway, FLOW-3D, Flow simulation, Nazloo Dam Spillway.