AbstractThe incorporation of fiber into cementitious composites is considered a reassuring way to enhance its mechanical properties. However, high-temperature exposure can severely affect it. This investigation aims to study the high-temperature behavior of cementitious composite containing steel and glass fibers from microstructure to mechanical properties. To examine the compressive and tensile strength, the specimens were heated to the target temperature (200°C, 300°C, 400°C, 650°C, and 800°C) and tested in the hot state. The volume fraction for steel fiber was 0.25% and 0.5%, and for glass fiber, it was 0.25%. Moreover, to examine the microstructural transformation, scanning electron microscope, X-ray diffraction, and thermogravimetric analyses were conducted on the samples extracted from the heated specimens. Based on the results, the mortar containing steel fiber enjoyed better mechanical strength at high temperatures. The inclusion of steel fibers could improve the compressive and tensile strength of the normal mortar, on average, by 9% and 14% at 400°C. However, the excessive inclusion of those adversely affected the compressive strength, particularly at temperatures above 650°C. Moreover, all types of mortars experienced a fluctuation phase in their mechanical strength at the range of 28°C–400°C resulting from rehydration of portlandite and evaporation of free water. The main reduction branch commenced at temperatures above 400°C, where decomposition of portlandite and microcrack development were noticeable.