In a method of diagnosing an RF generator of a laser produced plasma extreme ultra violet (LPP EUV) radiation source apparatus, a testing system is connected to the RF generator of the LPP EUV radiation source apparatus. An output power is measured by the testing system with changing an input power of the RF generator. Using a computer system, the measured output power is analyzed. Based on the analyzed measured output power, whether the RF generator is operating properly is determined.