This paper presents the mechanical properties of high strength structural steel and mild structural steel at elevated temperatures. Mechanical properties of structural steel at elevated temperatures are important for fire resistant design of steel structures. However, current design standards for fire resistance of steel structures are mainly based on the investigation of hot-rolled carbon steel with normal strength, such as mild steel. The performance of high strength steel at elevated temperatures is unknown. Hence, an experimental program has been carried out to investigate the mechanical properties of both high strength steel and mild steel at elevated temperatures. The high strength steel BISPLATE 80 (approximately equivalent to ASTM A 514, EN 10137-2 Grade S690Q, and JIS G 3128) and the mild steel XLERPLATE Grade 350 (approximately equivalent to ASTM 573-450) were tested using steady and transient-state test methods. The elastic moduli and yield strengths were obtained at different strain levels, and the ultimate strength and thermal elongation were evaluated at different temperatures. It is shown that the reduction factors of yield strength and elastic modulus of high strength steel and mild steel are quite similar for the temperature ranging from 22 to 540°C. The test results were compared with the predictions obtained from the American, Australian, British, and European standards.