An X-band high efficiency power amplifier with the highest PAE is presented in this letter. The single-stage power amplifier is implemented in TSMC standard bulk 0.18-μm 1P6M CMOS technology. In order to obtain wide bandwidth at PAE and output power, broadband output and input matching network are adopted in the design. From the measurements, the power amplifier obtained the best PAE of 25.8% and saturation output power of 23.8 dBm at 9.5 GHz. Besides, this PA demonstrates a 1-dB power bandwidth from 7.8 to 11 GHz and the PAE within the band all exceed 20%. To our knowledge, this power amplifier has the highest PAE, the smallest chip size, and wide bandwidth of output power and PAE in CMOS amplifiers at X-band to date.