Improved cooling design was verified of the reliability through the numerical analysis in order to develop the cylindrical high voltage generator. This study has investigated the temperature rise of rotor and stator windings with and without applying an air guide at NDE(non drive end). The air guide serves to efficiently maintain the structure of internal ventilation circulation, and local temperature variation may occur at he that the average temperature of the rotor windings decreased by about 1.9 °C and the average temperature of the stator windings increased by about 2.4 °C when the air guide is applied. The result of the reliability test showed the satisfaction of the insulation calss F (155 °C).