The goal of this research is to evaluate the influence of steel fibres and expanded vermiculite on some physical and mechanical properties of lightweight mortar at elevated temperatures. An experimental program has been carried out on lightweight mortar specimens exposed to high temperatures up to 1100 degrees C, combining different concentrations of both, steel fibres as a reinforcement of the mortar and expanded vermiculite. The target properties of the mortar to be determined were the flexural strength, the compressive strength and the ultrasonic pulse velocity of hardened mortar after air cooling. Non-linear models derived from a regression analysis and 3D graphics for the measured properties with good agreement to the test results are also presented. Lightweight mortar with expanded vermiculite shows a good performance at elevated temperatures that improves as the percentage of added fibre increases.