In this paper, nonlinear free vibration of double walled carbon nanotubes (DWCNTs) embedded in an elastic medium with geometric nonlinearity and interlayer van der Waals force nonlinearity are studied. The motion of the DWCNT is represented by multiple eigenfunctions of the linear system which are referred as trial functions. Describing function method (DFM) is employed in order to represent the nonlinear forces as a multiplication of a nonlinear stiffness matrix and a displacement vector, which made it possible to identify when it is necessary to consider multiple trial functions. The results revealed that it is important to consider multiple trial functions, especially in case of out-of-phase vibrations of DWCNTs with interlayer van der Waals force nonlinearity. The effects of number of trial functions and medium stiffness on the free vibration of DWCNTs are investigated. It is observed that utilizing multiple trial functions made it possible to detect different vibration modes occurring at a single vibration frequency. (c) 2012 Elsevier B.V. All rights reserved.