A linearity enhancement technique to a CMOS triple cascode push-pull power amplifier (PA) by second harmonic feedback has been proposed. The Second harmonic feedback is optimized to improve linearity based on the concept that the linearity is depend on the second harmonic feedback amplitude and phase. In order to feed second harmonics back to the push-pull PAs, we adopt a balun with a branching filter fabricated by using multi-layered organic substrates. The PA was fabricated in 0.18-μm CMOS process and was implemented on the balun by flip-chip connection. Because of the low breakdown voltage of CMOS power transistors, triple cascode configuration was adapted. It has been demonstrated that using second harmonic feedback, PAE and output power with a wideband code division multiple access (WCDMA) modulated signal are improved by 5.3% and 2.5dB respectively at adjacent channel leakage power ratio (ACLR) of -39dBc, and it has been demonstrated PAE of 28.8% and output power of 24.5dBm, respectively.