This paper is concerned with an efficient formulation of a planar variable geometry truss (VGT) . Since the VGT has many topological closed-loops, the number of generalized coordinates is less than that of the members. By using a Lagrangian method, an efficient formulation of the inverse dynamics is derived, because the derived equations of motion have a recursive form. Furthermore, the equations have contained the geometric constraints due to the topological closed-loops. The proposed method needs a less number of calculations in comparison with that of the Newton-Euler method. Effectiveness of the proposed method for inverse dynamics is examined by a numerical simulation.