A high‐resolution, high‐sensitivity, automated ac calorimeter capable of heat capacity measurements on very small samples (m<20 mg) with a temperature resolution of a few mK in the ac mode is described. This calorimeter also can be operated in the relaxation mode to provide absolute heat capacity values with precision and accuracy of around 2%. A microprocessor was employed to generate a stable oscillatory heating signal and to control the heater power, in order to improve the sensitivity of the measurement over other designs. A new very sensitive and miniature temperature probe (a film flake of a thermistor material), which increased measurement resolution and minimized the heat contribution of the addenda, also was used. This calorimeter was tested by measuring the heat capacity of gadolinium over its ferromagnetic phase transition. The results agree well with the literature data giving, however, substantially better resolution of the heat capacity in the critical region.