A method is developed for optimizing the size of a tension leg platform (TLP) and its tethers so as to minimize the weight-based cost of the TLP system subject to various design constraints. The method is intended for preliminary design studies of feasibility and cost sensitivity. Design constraints include minimum tension and maximum tensile stress in the tethers, maximum platform offset under current and waves, and minimum fatigue life of the tethers due to resonant vibration. Other given quantities include deck load, riser pretension, tether properties, and basic results from structural design studies and hydrodynamic analysis or model tests for a specific class of platforms. Example in TLP design studies are presented to illustrate application of the constrained optimization method.