Plasmonic nanocircuits have the potential to open up new routes in manipulating optical information beyond the diffraction limit and future quantum information processing technologies. We present on-chip polarization conversion based on interaction between two interfering surface plasmon modes supported by metal-insulator-metal (MIM) waveguides. The functional device is realized by all-plasmonic Mach-Zehnder Interferometers (MZI) equipped with impedance-matched Yagi-Uda style nanoantennas for highly efficient far-field coupling. By controlling the relative phase difference between guided MIM gap plasmons propagating in a MZI, namely by precisely differentiating the individual path length, a rotation of the mode optical axes is observed. Depending on the phase difference the resulting mode at the junction point, where two branching channels merge into one channel, can be symmetric or antisymmetric. Accordingly, two cases are investigated in ultra-compact (< 40 μm2) high-definition circuits, where the antisymmetric and symmetric mode are distinguished by observing the origin position and the polarization of the scattering signals. A high conversion efficiency can be realized, encouraging potential application in functional plasmonic nanocircuits.