A building structure, or parts of it, can fail in a fire situation if not designed to it. In fire events, the load-bearing behavior of concrete block structures has most times evidence an adequate performance. The behavior of modern concrete block masonry at elevated temperatures and the structure response in a fire situation is not fully understood, especially for high strength blocks. In many places in the world, a concrete wall is protected by a layer of cement-based rendering mortar. This work aims to investigate how the presence of a rendering mortar layer may influence the behavior of load-bearing concrete block masonry under fire situation. Finite element modelling with the Abaqus© package allowed to obtain isotherms curves for concrete block walls, considering a rendering layer of 20 mm, 10 mm, in the cold side of the wall, or without render. Heating conditions are assumed to be those of a standard temperature-time curve (ISO 834–1), from one or both sides of the wall. The results demonstrate that render thickness have a significant influence in the response of concrete block walls during a fire.