The load bearing capacity of steel structures after fire reduces mainly due to the deterioration of its mechanical properties. This paper presented an experimental study on material properties of high strength steels S460 and S690 after fire. The post-fire elastic modulus, yield and ultimate strengths, ductility and stress-strain curves were obtained and compared with the post-fire mechanical properties of mild strength steels. It is found that when the fire temperature from which the high strength steel is cooled down is less than or equal to 600 ℃, S460 and S690 can regain their original mechanical properties. When the fire temperature is up 1 000 ℃, S460 can also regain more than 75% of its original mechanical properties, but S690 only regains 64.5% of its original elastic modulus, 38.1% of its original yield strength and 57.3% of its original ultimate strength. The mechanical properties of high strength steels are different from mild strength steels. Predictive equations were proposed to evaluate the post-fire material properties of S460 and S690. Comparing with the test results, the predictive equations were validated. The predictive equations can be employed for practical inspection and appraisal of steel structures after fire with members made of S460 and S690, and provide a reference for the revision of current worldwide leading design standards.