A modified Lindstedt-Poincare (L-P) method for extending the validity of perturbation expansions to strongly non-linear oscillations of two-degree-of-freedom (DOF) systems is presented. A parameter transformation alpha = alpha(epsilon, omega(0), omega(1)) is adopted such that a strongly non-linear system with a large parameter epsilon is transformed into a small parameter system with respect to alpha: A typical cubic non-linear system in the form of a clamped-hinged beam is used to show its essential features. Three examples are presented to show the efficiency and the advantages of this method. (C) 1996 Academic Press Limited