The value of Beff for the TRIGA Mark III Reactor, belonging to the National Institute of Nuclear Researches (ININ), is reported. The TRIGA Mark III reactor core was simulated with MCNP6 to deduce the keff , for critical state and after a small insertion of positive reactivity (~ 0.20 $). To perform more realistic simulations, we had to incorporate in the composition of the LEU fuel element the produced poisons in a time period of six years, considering the operation time in days, during which the reactor was operating at maximum power (1 MWth). The calculation of the Beff value was obtained with the keff results, calculated with the code, and the reactor periods measured experimentally . We also obtained directly the Beff value, through the KOPTS card of the MCNP6 code in order to compare both values.