Structural and electrical properties of an individual single-walled carbon nanotube (SWNT) was characterized by in-situ electrochemical surface-enhanced Raman scattering (EC-SERS) measurement. An isolated SWNT was supported on a well-defined Au nanodimer structure, which possesses a localized surface plasmon resonance (LSPR) field at the nanogap. Intense SERS spectra were measured for the SWNT at the gap region. The electronic structural change in an isolated SWNT by photo-indueed oxidation reactions was monitored as the function of the time and the light intensity. The results indicate that EC-SERS technique is a powerful tool for detailed analysis of the local density of the states under the excitation of SWNT by LSPR.