In order to improve the power-added efficiency (PAE) without increasing chip size，a novel power amplifier (PA) with folded proportional series combining transformer used for wireless radio frequency (ＲF) is presented． Due to the cross coupling coefficient introduced by the folded structure，the primary to secondary winding electromagnetic coupling is enhanced，thus the power delivery efficiency is improved． Using the proposed folded proportional combining structure，the power amplifier is designed and implemented in standard 0. 18 μm CMOS process at the operating frequency of 2. 4 GHz． The measurement results show that，with a 2. 5 V power supply， the PA can achieve a peak output power of 28. 3dBm and a maximum linear output power (P1-dB) of 27. 2dBm in high power mode，with PAE of 33. 5% and 31. 6%，respectively，and a P1-dB of 19. 8dBm with PAE of 24. 1% in low power mode． It is demonstrated that the proposed design is suitable for the complicated wireless system with high efficiency and multi modulation requirements．