The ESA/NASA joint LISA (laser interferometer space antenna) mission is designed to detect gravitational waves, which relies crucially on maintaining three-spacecraft constellation as close to an equilateral triangle with a designed distance as possible. Efforts have been made to achieve this goal by using various simplified models to make it easy to approximately solve the complex problem. In this paper, the problem is formulated as a nonlinear optimization problem using exact nonlinear Kepler's orbit equations. It is shown that the optimal solution based on the exact nonlinear Kepler's orbit equations gives a better solution than the previously obtained ones.