A high efficiency feedforward power amplifier (FFPA) with a series diode linearizer for cellular base stations is presented. In order to achieve the highest overall efficiency of an FFPA, an improved pre-distortion diode linearizer has been used and the bias condition of the main amplifier has been optimized. The optimum bias condition has been derived from the overall efficiency analysis of the FFPA with a pre-distortion linearizer. From measured overall performances of the FFPA, efficiency enhancement of the series diode linearizer has been verified. The developed FFPA achieved the efficiency of 10% and output power of 45.6 dBm at 10 MHz offset Adjacent Channel leakage Power Ratio (ACPR) -50 dBc under Wide-band Code-Division Multiple-Access (W-CDMA) modulated 2 carriers signal. This design method can be also used to optimize the source and load impedances condition of the main amplifier FET.