In this paper, the effects of the use of metakaolin instead of cement on the mechanical properties of mortars, high temperature, alkali-silica reaction, resistance to sulfate, absorption capacity and permeability were studied. In the study, 0, 5, 10, 15 and 20 % cement was replaced by metakaolin by weight to design five different types of mortar. 1-, 3-, 7-, 28-, 56-, 90- and 180-day compressive and flexural strengths and 28-day absorption capacity were determined. Similarly, compressive strength loss in mortar exposed to 200, 500 and for 3 h was determined. The resistance of mortars containing metakaolin to sulfate and alkali silica reaction was determined in accordance with ASTM C 1012 and ASTM C 1260 standards, respectively. It was found that the use of metakaolin had a positive effect on the mechanical properties of the mortars positively and that it reduced the absorption capacity and degrees of expansion caused by ASR and sulfate attack.