A geometrical model has been developed to optimize the microstructure of porous composite anode electrodes of solid oxide fuel cells. The model takes into account the coordination number theory applied to a random packing of binary particle mixture, together with the percolation theory introduced to provide percolation paths for electrons and ions. The predictions confirm that the effective triple-phase-boundary (TPB) index deduced from a geometric analysis depends on the pore size as well as the characteristic parameters including particle radius ratio, contact angle and composition ratio. The influence of pore size on the effective TPB index is particularly conspicuous, suggesting that the pore size optimization should improve the effective TPB index significantly.