In this study, the MAAP5 code was used as a tool to simulate a BWR6/MarkIII nuclear reactor and which was LOCA with different break sizes at the recirculation loops. By detecting the temperature difference between the two loops, and then the break size could be determined. The results were consistent with the previous experiments or simulations. The study could be suggested to the operators at the main control room (MCR), when they observed some syndromes of the data and they could preliminary estimate the size of the LOCA. Those could help the operators to determine a strategy to avoid the probability of severe accident.