In the previous literature about RF generator, Efficiency of output of RF generator can reach 60-70 percent. In this paper, a new 500W RF generator with high efficiency and high stability is designed and fabricated for plasma applications. The efficiency of a power amplifier in the RF generator is improved by using Class-E amplifier. The Class-E power amplifiers described here is based on a load network synthesized to have a transient response which maximizes power efficiency even if the active device switching times are substantial fractions of the AC cycle. For that circuit, the author measures 73 percent efficiency of output of RF generator at 13.56MHZ and 478W output from DE375 MOSFET. The main reasons of power dissipation mainly are analyzed, and some measures are taken to improve them by the optimization principles and experimental results and the efficiency of the power output can reach above 70 percent.