Nowadays, the use of high strength concrete has been increased. However, its dense microstructure leads to spalling due to fire, which causes degradation and collapse of concrete structures. Various attempts regarding optimum solution for spalling prevention of high strength concrete are made continuously. In this paper, tests are carried out to clarify the effect of elevated temperature on mechanical behavior and microstructure of the high strength concrete incorporating 20% of fly ash and 10% of silica fume. 0.05% of nylon and polypropylene fiber was added to the concrete to prevent a spalling during test. Temperature was varied from $20^{circ}C$ to $800^{circ}C$, with the interval of $100^{circ}C$. For the observation of microstructure of the concrete, SEM, MIP and XRD tests were conducted. According to results, An increase in temperature led to decrease of strength above $400^{circ}C$ and ductile behaviour between strain and stress relationship was observed at above $400^{circ}C$.