Dynamic stal behavour of a NACA0012 airfoil undergoing pitching motion has been studied by a numerical approach. The turbulence intensity, oscillation frequency and amplitude and the Reynolds number were found to be the major contributors in dynamic stall. The flowfield structure and the associated vortices for this airfoil as well as the impact of the oscillation frequency on aerodynamic efficiency were also studied. The simulations were two dimensinal and the k-ω SST turbulence model were utilized for the present analysis. The results show that increasing the oscillation frequency and amplitude and the turbulence intensity, postpones the dynamic stall to higher angles of attack. Furthermore, as increasing the Reynolds number, both the lift coefficient and the width of the associated hysteresis loop decrease. The airfoil aerodynamic efficiency variation with oscillation frequncy has been shown to have a maximum point for all angles of attack considered. The flowfield structure revealed that the main cause of the dynamic stall is a series of low pressure vortices formed at the leading edge which shed into downstream and separate from the surface. A secondary vortex will then appear and increases the lift coefficient dramatically. The present simulation results are in a good agreement whith those found in the literature.