This research presented the details of design and constructions of tesla transformer for testing insulator. The out put of the tesla transformer has a rate of 120 kV and 120 kHz. Tesla transformer consisted of primary coil and secondary coil which is wound on a PVC insulating chamber and use air insulation in part of coaxial. The Rotary Gap is rotated eletrode which use for increasing efficiency of voltage generation. This paper presented the details of design and constructions are explained. And compare the tesla Transformer reproduce which is created the voltage at 120 kV and frequency 120 kHz for used in HighVoltage Laboratory Faculty of Engineering Rajamangala University of Technology Para Nakhon.