In this work, we develop a new residue-based protein-surface interaction potential model. With this model, the adsorption and orientation of two antibodies, IgG1 and IgG2a, are studied by Monte Carlo simulations. Effects of surface charge density and sign, and solution ionic strength are examined in our simulations. Simulation results show that van der Waals and electrostatic interactions codetermine the orientation of adsorbed antibodies. At low surface charge density and high solution ionic strength, where van der Waals interactions dominate, both IgG1 and IgG2a exhibit multiple orientations. At high surface charge density and low solution ionic strength, where electrostatic interactions dominate, there are preferred orientations for these two antibodies on both positively and negatively charged surfaces, which are verified by experimental results. Due to a smaller dipole moment, IgG2a has more possible orientations than IgG1. IgG1 adsorbed on a positively charged surface shows an "end-on" orientation, which is well suited for biosensor applications. The simulation methodology and model could be directly applied to predict the adsorption and orientation of other proteins and to provide a fundamental understanding of their behavior on surfaces at the molecular level.