Based on the physical picture of nanoparticle-infiltrated electrode of SOFC, a new theory for the electronic conductivity of the electrode is developed. The theory provides a clear explanation for the observed conductivity below the conventional percolation threshold. The observed dependences of the electrode conductivity on the content of nano Ni particles as well as the operating temperature are well reproduced by the theory. The theory is also used in a multi-physics numerical model and excellent agreement between the theoretical and experimental IV curves is obtained. The new theory allows for in-depth analysis and design optimization of the nanoparticle-infiltrated electrode.