Normal form method is a classical ODE technique begun by H. Poincare. Via a suitable transformation one reduce a differential equation to a simpler form, where most of nonresonant terms are cancelled. In this talk, I begin to explain the notion of resonance and the normal form method in ODE setting and Hamiltonian systems. Afterward, I will present how we apply the method to nonlinear dispersive equations such as KdV, NLS to obtain unconditional well-posedness for low regularity data.