The objectives of this study are to clarify the dissolution behavior of the ferritic phase (α phase) and the austenitic phase (γ phase) of a duplex stainless steel (DSS), and to propose a corrosion mechanism of the DSS through dissolution behavior of either the α phase or the γ phase at corrosion potential, respectively. Just a single phase specimen (α or γ phase specimen) was prepared by using selective dissolution of the DSS. The α phase showed less-noble corrosion potential compared to the γ phase, and passivated at less-noble potential. When the polarization curves of each phase are summed up, the obtained curve showed good agreement with the DSS polarization curve. The α phase of the DSS dissolved preferentially at corrosion potential. Based on these results, a preferential dissolution mechanism model of a DSS is proposed using internal polarization curves of each phase.