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Composite insulation system of liquid nitrogen and solid insulator, which are widely employed in high TC superconducting equipment, can be deteriorated by partial discharge (PD) generated in highly stressed region. By subcooling the liquid nitrogen below the boiling point of 77 K at atmospheric pressure, the superconducting power device can obtain higher performance because the critical current density of the superconductor can be increased. The subcooled liquid nitrogen may also have better properties as electrical insulation medium than pool boiling one because gaseous bubble generation is suppressed. In this study, effects of subcooling temperature on PD characteristics were investigated at atmospheric pressure. Behavior of PD induced bubbles was optically investigated using a high speed digital imaging system as well as a laser light scattering method. These optical observations were synchronized with PD signal detection in order to clarify relationship between the PD intensity and bubble behavior. It was found that PD induced bubbles were strongly influenced by the PD energy as well as temperature of the subcooled liquid nitrogen.