In designing a compact and high efficiency TWT microwave amplifier, the role of a high power density DC power supply is very important. In order to reduce volume of the power supply, it is necessary to increase switching frequency of the power supply. The high operation frequency mainly reduces transformer volume in the power supply. However, a step-up ratio of the transformer in the high voltage DC power supply needs to be maintained. Therefore, the pulse transformer should be carefully designed to fulfil its function in the power supply. A high frequency and high voltage pulse transformer is designed, fabricated, and tested. Switching frequency of the transformer is 100 kHz. Input and output voltages of the transformer are 250 V and 4 kV, respectively. Normal operation power of the transformer is 3 kW. Maximum volume of the transformer is 400 cm/sup 3/. The power density is thus 7.5 W/cm/sup 3/. The transformer will be installed in a metal box that has nominal operation temperature of 85 degree centigrade. The transformer and other high voltage components in the box will be molded with silicon RIV that has a very low thermal conductivity. Procedures of design and test results are discussed. Analytical as well as experimental results of various parameters such as transformer loss, temperature rise, leakage inductance, and distributed capacitance are also discussed. In addition, thermal analysis results from ANSYS code for three different operation conditions are discussed.