We present an optical slot antenna integrated with a metal-dielectric-metal (MIM) plasmonic waveguide. By integrating optical slot antenna on top metal layer of MIM waveguide, we can couple the plasmon guide mode into the feed antenna directly. The resonantly excited slot antenna works as a magnetic dipole and then radiates in dipole-like far-field pattern. By adding an auxiliary groove structure along with the slot antenna, the radiation can be directed into the direction where the structure determined. The demonstrated optical slot antenna integrated with a plasmonic waveguide can be used as a "plasmonic via" in plasmonic nanocircuits.