Just as scalar potential theory can be applied to magnetostatic field, where vector potential theory is the dominating theory, so may vector potential theory be used in steady current field computations, dominated by scalar potential theory. The introduction of vector potential theory simplifies the computation of spontaneous potential (SP) and brings about the innovation of concept. Finite element method is applied to the computation based upon both vector potential theory and scalar potential theory. Their results agree very well with each other. SP correction charts computed on vector potential agree with existing ones.¿ 1997 American Geophysical Union