This study investigates the effect of elevated temperature on cementitious composites with 5, 10, 15, 20, and 25% firebrick powder (FBP). In this regard, cementitious composite mortars with di-mensions of 40 x 40 x 160 mm, which were water-cured at 20 +/- 2 degrees C for 56 days, were pro-duced. Produced samples were exposed to 300, 600, 750, and 900 degrees C, then air-and water-cooling processes were applied. Cooling processes were continued until the samples reached the labora-tory temperature. Unit weight, ultrasonic pulse velocity, compressive strength, and mass loss of the FBP-based cementitious composite samples were measured after both air-and water-cooling regimes. Finally, microstructural analysis was performed for reference samples and samples with 20% FBP. As a result, the compressive strengths of the samples exposed to 600, 750, and 900 degrees C showed that up to 15% of FBP-based cementitious composite samples had better results com-pared to the reference samples. The compressive strengths of the water-cooled samples were lower than those of the air-cooled samples. The mass loss results of FBP-based samples decreased with the increase in temperature. The mass loss of the air-cooled samples was higher than that of the water-cooled samples.