This paper deals with an optimal ZMP based pattern generator for stable dynamic walking. The proposed method is based on a Three-Mass Linear Inverted Pendulum Model (3MLIPM), used as a simplified dynamics of the biped robot. The 3MLIPM simplifies the biped robot as a three point masses and two-link system. A ZMP based criterion is then used in an optimization problem whose solution gives the best values of the model's parameters w.r.t. dynamic walking stability. Numerical simulations are presented to show the effectiveness of the proposed optimal pattern generator.