This paper presents the results and methodology used to calibrate the thermal power of the TRIGA Mark I IPR-R1 Research Reactor at the Nuclear Technology Development Centre (CDTN), in Belo Horizonte, Brazil. The TRIGA Mark I is a pool type reactor, cooled by water natural convection. The method used in the calibration consisted in the steady-state energy balance of the primary cooling loop of the reactor. For this balance, the inlet and outlet temperatures and the water flow in this primary cooling loop were measured. The heat transferred through the primary loop was added to the heat leakage from the reactor pool. The thermal losses from the primary loop were not evaluated since the inlet and outlet temperatures were measured just above the water surface of the reactor pool. The temperature of the water in the reactor pool as well as the reactor room temperature were set as close as possible to the soil temperature to minimize heat leakages. These leakages are mainly due to the conduction through the concrete and metal walls and also due to the evaporation and convection through the water surface of the reactor pool.