Ultra-small nanogap of plasmonic metal dimer can confine light into molecular size. In this study, we have tried to establish the method for the control of the gap distance of Au nanodimer structures using electrochemical method. Au nanodimer structures were fabricated on a conductive glass substrate by the electron beam lithography method. Electrochemical dissolution of bridged Au nanodisk dimer was conducted under in-situ dark-field microscopy observation. The electrode potential was kept at the potential for Au dissolution. SEM images proved the formation of the gap between Au nanoparticles. From the time series scattering spectra of Au nanodimer, the drastic optical property changes has been observed at the gap formation. This optical property change can be assigned to the plamon mode change from the charge transfer mode to bonding dipolar mode due to the formation of the sub-nm gap at Au nanodimer. The present method is a useful tool for controlling the shape and the gap distance between Au nanoparticles in a single nanometer scale.