An X-Band 14W high efficiency internally-matched HFET has been developed. In order to achieve high efficiency, not only equal combining characteristics but also equal load impedance for each unit FET cell are designed in the input and output matching circuits using EM simulator. In addition to the power splitting and combining characteristics, the large signal load impedance of each FET cell is discussed. The degradation of the efficiency by unequal operation of each FET cell and by mismatch to the optimum load impedance are suppressed within 1% and 2%, respectively in the design. The developed HFET has achieved a power-added efficiency (PAE) of 49.8% and an output power of 41.6dBm(14.5W) in X-band.