In the present paper, the effect of using multi-walled carbon nanotubes (MWCNTs) with different concentrations of 0, 0.05, 0.1, and 0.15 wt% on the mechanical properties and microstructure of alkali-activated slag (AAS) mortars at temperatures of 23, 200, 400, 600, and 800°C was investigated. In order to investigate the strength parameters of specimens, the mechanical strength and mass-loss of the specimens at ambient temperatures and after exposure to elevated temperatures were determined. Petrographic image analysis also was conducted to analyze the microstructures of the specimens. Moreover, the Image processing toolbox of Matlab software was used to calculate the pore area of the samples at room temperature. The results showed that the application of MWCNTs improved the mechanical properties of the AAS mortars at ambient and elevated temperatures. The addition of the MWCNTs improved the mechanical properties of the AAS mortars up to 69 and 85% for compressive and flexural strength, respectively. At 400°C, compressive strength increment of the specimens compensated the deterioration rate, which was about 3% mass-loss and this temperature should be considered the maximum tolerable degree for AAS mortars.