There are inconsistent results in previous studies that focused on the effect of elevated temperatures on metakaolin (MK) geopolymers. Likewise, some studies reported a positive effect of silica fume (SF) on the compressive strength of MK geopolymers, while others reported a negative effect. In this paper, the authors tried to get suitable reasons for these contradictions. Therefore, two different concentrations of sodium silicate and different ratios of SF that affect the Si/Al and Na/Si molar ratios were used. For each concentration of activator, MK was partially replaced with 5 to 25 wt.% SF. The flowability was measured and the compressive strength of the specimens before and after exposure to 400 to 1000°C was recorded. The results were analyzed by different techniques and showed higher flowability with the inclusion of SF. The incorporation of SF may increase or decrease the compressive strength before and after heating, depending on activator concentration—that is, Si/Al and Na/Si.