Abstract: The forced oscillation of Duffing equation is studied by a variational iteration method. As a result, a uniformly valid asymptotic approximation in the entire region (0 < ε < ∞) is obtained, the maximal relative error of the period obtained is less than 5.66%, and the obtained primary resonance and superharmonic resonances are also uniformly valid in the entire region.

Key words: Perturbation Method, variational iteration method, duffing equation