A 2.6-GHz power amplifier was implemented with a 0.18 μm RF CMOS process. The matching networks were fully integrated without any external components. To increase the output power, the power combining technique was adopted and realized by a 2:1 on-chip transformer, which was designed to provide an optimum load for the power cell. The measured efficiency of this power combining transformer is about 75% at 2.6 GHz. The proposed power amplifier exhibits maximum output power of 26.7 dBm with power-added efficiency of 18.1%. The power gain is 13.6 dB under 3.3 V supply voltage. Moreover, this letter shows good agreement between simulated and measured results.