The microwave heating system of KSTAR consists of ECH and LHCD. ECH and LHCD offer reliability of operation at the beginning of plasma formation and non-inductive current drive for long-time steady-state operation, respectively. LHCD demands a C-band microwave system with a frequency of 5 GHz. A pulse generator with a power of 3.6 MW, 4 μs, 200 pps is required to drive a high-power magnetron. The high power pulse transformer has the function of transferring pulse energy from a pulsed power source to a high-power load. A pulse transformer producing a pulse with a peak voltage of 45 kV is required to produce a 5 GHz microwave source in a 1.5 MW magnetron. We have designed the high-power pulse transformer with 1 : 4 step-up ratio. The peak power handling capability is 3.6 MW (45 kV, 90 A at load side with 4 μs pulse width). In this paper, a system overview and test results of the high power pulse transformer are presented.