Boiling heat transfer characteristic of liquid nitrogen was investigated to obtain the basic information on cooling design of high-TC superconducting transformer which is cooled by subcooled nitrogen to avoid bubble formation. Thin wire of platinum-30%/rodium in diameter of 50μm was used as heater and thermometer. The wire was covered outside with acrylic tube to simulate the practical winding condition and we examined the effect of space restriction on boiling heat transfer characteristics. The subcooling of nitrogen tested was between 0 and 10.4K. The results show that the CHF and MHF increase considerably with the degree of subcooling.