The present study aims to investigate the lock-in phenomena due to the coupling of the pitch and the plunge motion with vortex shedding from an airfoil modeled using the nonlinear van der Pol oscillator. The coupled two degrees of freedom (dof) motion in plunge and pitch of an airfoil approximates the vibration of a turbomachinery blade. The amplitude and the frequency of the blade motion will modify the van der Pol oscillator. Such effect of the blade on the wake dynamics is considered through a linear inertial coupling. On the other hand, the periodic vortex shedding exert lift and moment on the blade. The actions of the blade on the wake and the aerodynamic lift and moment from wake dynamics on the blade form the three dof Fluid-Structure Interaction (FSI) problem. Coupled three dof FSI equations are then expressed in terms of the identified non dimensional quantities. The equations of motion are solved to investigate the lock-in phenomenon of an airfoil. The influence of the different dimensionless parameters such as mass ratio, the linear inertial coupling parameters, the structural damping of blade, nonlinear fluid damping in the fluid oscillator and the reduced flow velocity on the range of lock-in and the amplitude of oscillation of the airfoil is studied. © 2019, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.