A three-dimensional numerical simulation of the solid oxide fuel cell (SOFC) anode overpotential is conducted in a microstructure which is reconstructed by dual-beam focused ion beam-scanning electron microscopy (FIB-SEM). Gaseous, ionic, and electronic transport equations are solved by a lattice Boltzmann method with electrochemical reaction at the three-phase boundary. The predicted anode overpotential agrees with the experimental data at the fuel supply of 1.2% H2O-98.8% H-2, while it is larger than the data at 10% H2O-90% H-2. The dependence of exchange current density on steam partial pressure, gas diffusion modeling, as well as computational domain size must be further investigated in the future. Local three-dimensional distributions of electrochemical potential and current density inside the anode microstructure are obtained. Their nonuniformities are attributed to the scattered three-phase boundaries and complex transport paths through the solid phases.