The optical phase change associated with the surface plasmon resonance (SPR) effect that exists in a glass-metal- dielectric stack has been studied using a differential phase imaging technique. A typical prism-coupled SPR setup was constructed and a Mach-Zehnder interferometer was used to perform interferometric analysis between the two orthogonal polarizations in the exit beam. By stepping the optical phase of the reference arm, one can measure the phase change caused by the SPR effect. Since the reference and signal beams traverse identical optical paths, we expect that this scheme can be more robust in terms of noise immunity. The interrogation area can be enlarged to enable imaging of the SPR sensing surface. Initial phase measurement obtained from a salt-water mixture will be presented to demonstrate the operation of the technique.