The efficiency of microwave PAs is limited by the active device parameters and operating conditions. High efficiency can be obtained by a proper selection of bias point and harmonic terminations or, from a different point of view, by a proper output voltage and/or current waveform shaping. This work describes the design of a high efficient 3.5 GHz GaN HEMT power amplifier which may be used in WiMAX applications. The PA is designed by using a 2nd harmonic tuned (HT) approach and has been realized and measured. Its measured performances confirm the improvement obtained by means of harmonic manipulation over class AB (TL PA). An output power of 35.3 dBm has been measured for the maximum power added efficiency of 57.7%, drains efficiency of 69.18% and measured C/I3 of 19.1 dB.