In this paper, the authors deal with the possibility of nonlinear excitation in “Tension Leg Platform” (TLP) which is categorized into vertically moored floating platform. The authors formulate the governing equation of sway and surge motions of TLP in which case TLP was modeled for mass-spring system with the consideration of geometric stiffeness effect, and carried out the fundamental test using one taut moored floater model. Principal findings are as follows; 1) The motions of TLP generated by wave are expressed with Mathieu type differential equation. 2) Parametric excitation suggested from theoretical consideration was qualitatively confirmed from fundamental tests. 3) For the more plactical estimation of stability range and steady-state amplitude, more detail examination of hydrodynamic force are necessary.