This work aims to prediction of laminar/turbulent transition which plays an important role on aerodynamics of wing section. In this respect the flow around the NACA2415 airfoil simulated in a Computational Fluid Dynamics (CFD) solver in different regimes with and without propeller flowfield. For predicting the transition onset, two approaches were used: The first is based on time history of the skin-friction coefficient for determining the transition onset and the transition length on the airfoil. The second is to apply transition γ-〖Re〗_θ model for laminar/turbulent transition simulation. For investigation of transition effect, the simulation repeated by use of a classical turbulent model and both results was compared with experimental data. The comparison shows that taking into account the transition effects gives a good agreement with experiment. Relative error of calculated drag coefficients for the transition based simulation is lower than 10%, while fully turbulent simulation are 70% overestimated in some incidences. Slipstream of upstream propeller changes flow pattern and boundary layer characteristics over the wing. Indeed in presence of propeller, spanwise load distribution and laminar/turbulent transition onset were affected. In propeller flowfield, increasing of velocity normal component over wing surface causes transition delay. Movement of transition onset to trailing edge on the upper surface in propeller downwash is representative of such phenomenon. On the other hand, in upwash region, the transition onset moves upstream. With the increasing propeller rotational speed, this tendency augments and so the transition onset on the wing upper surface moves far downstream in propeller downwash.