In this study, physical and mechanical properties of light mortars produced with Fly Ash (FA) contributed pumice aggregates, Colemanite (K), Blast Furnace Slag (BFS), Marble powder (MP) were investigated under high temperature. Mortar samples were produced in the dimensions of 40x40x160 mm. At the preparation of mortars, pumice (0-4mm) as a fine aggregate and CEM I 42.5R Portland Cement were replaced with. 1%, 2% and 3% by weight of K respectively and 10%, 20% and 30% of YFC, MT and UK were replaced with cement in light mortars mixture. The produced mortar samples were removed from the mold after 24-hour setting and subjected to a cure at a temperature of 20±2°C in the standard cure pool for 28 days. The flexural and compressive strength of the mortar samples filling the 7th day were determined. Physical properties such as water absorption, porosity and unit volume weight of 28 days cured mortars were determined and flexural and compressive strengths were calculated. At the end of 28th day samples, which were reached its final strength, were exposed to 200 ⁰C, 400 ⁰C, 600 ⁰C and 800 ⁰C heats in High Temperature Oven. It is observed that with increasing temperature, weight loss increases, flexural and compressive strengths decrease in all samples. With the use of mineral additives, it was observed that both the weight losses and the losses in flexural and compressive strengths decreased and the mortars became more resistant to temperature.