Dynamic stall is a complex aerodynamic phenomenon that occurs from the unsteady flow around horizontal axis wind turbines resulting in amplified aerodynamic loads and severe blade stresses. Numerical methods are computationally expensive but can model the flow physics during dynamic stall more accurately and overcome the limitations of many existing semi-empirical and analytical models. In the present study Large Eddy Simulation (LES) based CFD approach together with a sliding mesh technique was implemented to accurately model the unsteady aerodynamics around a large flexible blade of a 3-bladed rotor. The 5 MW NREL wind turbine’s baseline rotor was used to develop the detailed geometric and structural models. The influence of various factors like transient wind shear, unsteady wake behaviour and blade flexibility on the complex flow physics during 3D dynamic stall has been studied. Factors like wake turbulence and blade oscillations amplify the unsteady effects leading to dynamic aero-elastic instabilities like flutter and cause the blades to fracture. The study aims to provide the complete CFD investigation of events happening during dynamic stall occurrence on the rotating wind turbine blade. Simulations using 3D models are important for accurate estimation of fatigue life and determining the structural dynamic response of the turbine blades. © 2016, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.