Abstarct: Turbulent flow inside square ducts represent important engineering issues, which are characterised by the existence of secondary flows. A phenomenon not observed in circular ducts nor in fully developed laminar flows through straight rectangular ducts. Many researches about secondary flow in non-circular duct is conducted for square duct. In this research, fully developed turbulent flow in non-circular straight ducts with square, rectangular, triangular and regular polygon of cross sections has been simulated using the large eddy simulation for an incompressible flow and the presence or absence of secondary flows is investigated. In this model Navier-Stokes Equations using the finite volume method on non-uniform grid for the entire field using Smagorinsky model will be considered. The secondary velocities are usually about 0.5%-1% of the streamwise bulk velocity in magnitude but appreciably alter the rates of momentum and heat transfer near the walls. This secondary flow has been thought to be arising due to the anisotropy of the Reynolds stresses. There is no secondary flow in circular duct. Therefore in second section of this research, the corners of these sections is rounded and step by step round radius is increased and cross section likes circle, and the presence or absence of secondary flows is investigated. Flow simulation is performed using OpenFOAM software. PisoFoam solver is used in this simulation that is created for single-phase and incompressible fluid. Numerical solution methods used in pisoFoam solver is baxsed on PISO algorithm.