A numerical study was conducted to identify the contributions of curvature to the displacement speed of turbulent premixed flame by using the governing transport equation for the deficient reactant. The displacement speed was shown to be a balance of three components: reaction, normal diffusion, and curvature. The contribution of the three components was evaluated using results from direct numerical simulations (DNS) of an unsteady stoichiometric methane-air flame embedded in a 2-D vortical flow field.