Micro gas turbines fit perfectly with the energy roadmap to 2050: on-site, small scale power generation, combined with heat recovery from exhaust gas, offers an opportunity to deploy primary energy saving and pollutant emission reduction. Moreover, their flexibility enables fuel switching from natural gas (NG) to carbon-free fuels such as hydrogen and ammonia. This study aims to explore the potential of direct combustion of ammonia in a micro gas turbine (MGT), from a thermodynamic point of view. A modeling procedure was developed to simulate the behavior of a 100 kW MGT operating at full and part-load. After validation with NG as fuel, an increasing fraction of ammonia was fed to the combustor to predict performance variations in terms of electric, thermal and total efficiency, as well as exhaust gas composition, for a load range between 40% and 100%. Additional relevant details, related to the interaction between compressor and turbine in the single-shaft arrangement, were discussed through performance maps. Full replacement of NG with ammonia was found to reduce electric efficiency by about 0.5 percentage points (pp), whatever the power output, with a consequent improvement in exhaust gas heat recovery. Thus, total efficiency is maintained at a high level, with values ranging from 74.5% to 79.1% over the investigated load range. The benefit of zero CO2 emissions can be achieved provided that compressor–turbine matching is adjusted to compensate for the reduction in fuel calorific value: at rated power, when the largest fuel input is required, flow rates of air and flue gas decrease by 4.3% and 2.8%, respectively, with an increase in Brayton cycle pressure ratio of 2%.