This paper presents the improvement of efficiency of power amplifiers by 2nd and 3rd harmonics injection with a newly developed triplexer. An L-band 10W GaN HEMT PA with input and output triplexers have been fabricated. The dependence of injection power level on the improvement of efficiency from both input and output injection is studied. The 2nd harmonics injection delivers higher efficiency improvement than that of 3rd harmonics injection. The best injection power of 2nd harmonics for improvement of extended power added efficiency (including additionally harmonics injection power as input power) is 20.5dBm and 26.5dBm for input and output injection, respectively. The PA has achieved the excellent improvement of 10.6% by both input and output 2nd harmonics injection simultaneously. The maximum extended power added efficiency of 64.7% with an output power of 39.9dBm is obtained at 1.5GHz.