In this paper, well-known component mode synthesis technique, which developed by Craig and Bampton, has been applied to substructure synthesis of a geometrically nonlinear beam. In order to consider the geometric nonlinearity, a reduction basis(or CB transformation matrix) has been formed to include the modal directional derivatives of linear vibration modes(VMs). All the VMs and their derivatives are calculated once at the undeformed configuration, and they are not updated. Various numerical examples are tested to assess the proposed approach.