Development principles of an insulation structure for a prototype UHV transformer are presented. New interleaved windings having gradual apportionment of series capacitances were developed, and shown to have approximately uniform voltage distribution for lightning impulse and high reliabaility in their dielectric characteristics. The insulation structure developed for the UHV transformer was applied to a 500 kV transforrner, which allowed its floor space, weight and loss to be reduced about 20 % compared to a conventional 500 kV transformer.