The physio-mechanical properties of sandstone are subject to considerable changes when undergoing heat treatment. Apart from the inherent geothermal gradient, abrupt heating of rocks can be caused during underground coal gasification, nuclear waste disposal and building fire, prompting research surrounding behaviour of rocks when exposed to higher temperatures. In this study, mechanical behaviour of sandstones with different grain sizes was subject to higher temperatures to investigate the changes in their mechanical behaviour. Four distinct varieties of Dholpur quartz-arenitic sandstone, which were exposed upto a maximum temperature of 900°C at the rate of 5°C/min rise. These samples were therefore examined at 35, 100, 300, 500, 700 and 900°C. The compressive and tensile strength of the sandstones shows an inverse relation to the percentage of quartz in a heating environment, irrespective of the grain sizes. Moreover, the value of ultrasonic wave velocity, Poisson’s ratio and elastic modulus gradually decrease for the sandstones up to 500°C but sharply fall afterwards. The differential thermal analysis (DTA) indicates that, compared to the oxidised environments, the endothermic reaction causes a relatively low mass loss in the atmospheric conditions for the sandstones with an abundance of Hematite and Kaolinite Minerals. Hence, the overall analysis concludes that the variation in mineral assemblages and grain sizes influences the impact of heat on the geomechanical strength of the sandstone rock.