The effects of vortex formation on the performance of orifice spillways using two physical models (sectional and comprehensive) of the Karun III dam in Iran, have been studied and presented in this paper. The sectional model was designed with one orifice and a scale of 1:33.33, and the scale of the comprehensive model was selected as 1:80. Experiments on the sectional model showed that in relative submerged depth (S/D) = 5.6 a weak vortex (Class C) was formed. By decreasing S/D to 3.7, a strong (Class A) vortex was formed. At S/D = 2.4 vortices again became weaker until they were eliminated when S/D = 1.2. Also, where S/D = 3.7, a stable vortex was formed whilst in other S/D’s all observed vortices were unstable. The experiments also show that in presenting vortices with different classes, the discharge coefficient of the orifice spillway was decreased. The experimental results on the comprehensive model of the Karun III dam with four orifice spillways showed that operating them together has a positive effect on reducing vortex strength.