The development of different constructive techniques is somehow conditioned to the investment in scientific research, aiming at comprehend the behaviour of materials; not only as individuals, but also as a whole compound. One area that has growth, for its importance in structural integrity analysis, is the analysis of masonry structures in fire situations. However, there exists an inherent difficulty on performing real standard tests under fire situations. The present work has as its main objective the study of the thermal behaviour of clay blocks at high temperatures, having the geometry of internal cavities as the main reference parameter. For this purpose, four different geometric configurations were analyzed. The thermal analysis was accomplished by using the commercial software ABAQUS. It was considered radiation, convection and conduction as heat transfer mechanisms. Radiation and convection was accounted for heat transfer between fire and the fire-exposed face of the wall, and between the wall and the ambient. As a simplification, it was considered air as solid for the cavity representation. Thus, heat conduction was considered within the internal cavities. The numerical simulations were purely thermal, aiming at analyzing the dependence of the internal geometry of the cavities. Numerical results show that thermal behavior not only depends on the empty spaces, but also on their distribution along the block interior.