A novel linearization technique to a CMOS push-pull power amplifier (PA) by second harmonic injection has been proposed. In order to inject second harmonics to the PAs, a new balun with a branching filter has been proposed and 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. The proposed push-pull PA has delivered an output power of 28.6 dBm with a power-Added efficiency (PAE) of 42.2 % at 2.0 GHz and 5.4 V supply voltage. It has been also demonstrated that using second harmonic injection from input port of the PA, PAE and output power with a wideband code division multiple access (WCDMA) modulated signal are improved by 5 % and 2.0 dB respectively at adjacent channel leakage power ratio (ACLR) of -39 dBc.