This paper reports the development of a high efficiency and compact power amplifier using an AlGaN/GaN high electron mobility transistor (HEMT). The class-F load network in microstrip topology was applied to a 0.75-μm gate length and 300-μm gate width AlGaN/GaN HEMT. The 1 dB compression point of the output power was 24 dBm at 900 MHz. The peak power added efficiency of 38% was optimized at output power of 24.5 dBm, which is almost the same as the 1 dB compression point. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 1955–1957, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21832