We made ESD tester to measure ESD threshold voltage of semiconductor devices. The HBM ESD test is the most popular method to measure the ESD threshold voltage of MMSIC. We use flyback method which is one of the DC-DC converter to get high ESD voltage. With flvback method, we can isolate the 1ow voltage part from the high voltage part of HBM ESD tester. We use an air gap of the relay which is used for switch to satisfy the rise time of ESD standard(MIL-STD). As a result, with the flyback method and the air gap of relay, we can make ESD tester whose parasitic components are minimized.