Fire hazard is one of the most severe conditions to which steel structures might be exposed to. In fireresistance design of steel structures, the deterioration of mechanical properties of steels under fire condition is of significance, as it leads to reduction of load bearing capacity of the structures. An experimental study has been conducted on high strength structural steel S690, under both steady state and transient state fire conditions, to reveal the deterioration of high strength structural steel S690 exposed to fire temperatures up to 700°C. Comparison of experimental results with current European, American, Australian and British design standards showed no current standard can be safely used to predict the performance of high strength structural steel S690 exposed to fire, because the recommendations of the leading standards on structural steels were obtained mainly from mild steels. In order to offer accurate choice for structural engineers to conduct safe fire-resistance design of steel structures with S690, specified predictive equations were developed and presented herein to calculate the reduction factors of mechanical properties of S690 exposed to various fire temperatures. The reduction factors can be used as input for numerical modeling of the fire performance of steel structures with S690, and can be used as basis to conduct relevant fire-resistance design as well.