Two normal strength concretes and three high strength concretes, with 28-day compressive strengths of 28, 47, 76, 79 and 94 MPa respectively, were used to compare the effect of high temperatures on high strength concrete and normal strength concrete. After being heated to a series of maximum temperatures at 400, 600, 800, 1000 and 1200°C, and maintained for 1 hour, their compressive strengths and tensile splitting strengths were determined. The pore size distribution of hardened cement paste in high strength concrete and normal strength concrete was also investigated. Results show that high strength concrete lost its mechanical strength in a manner similar to or slightly better than that of NSC. The range between 400 and 800°C was critical to the strength loss of concrete with a large percentage of loss of strength. Microstructural study carried out revealed that high temperatures have a coarsening effect on the microstructure of both of high strength concrete and normal strength concrete.