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High‐strength steels can be used in civil structures that demand high strength/weight ratios, but their advantages may be weakened by fire. For this reason, this study has been concerned with the mechanical properties of a type of quenched‐and‐tempered high‐strength steel at elevated temperatures up to 800°C, the steel grade is S690. A standard axial tensile test was conducted on plate specimens with a reduced section. The test results demonstrated that the stress–strain curves of S690 steel at elevated temperatures had no distinctive yield plateau but post‐yield hardening, the effective yield strengths and elastic modulus deteriorated dramatically with the increasing temperature due to microstructure changes. Besides, this study also investigated the effects of steel plate thickness on said temperature‐dependent mechanical properties. The plate thicknesses are 8 and 12 mm nominally. The comparisons showed that, different from the trend at room temperature where the thinner steel plates usually have higher strength than their thicker counterparts, the thinner plates exhibited lower strengths at elevated temperatures, this implies that with the same cross‐sectional area, a steel member formed by thinner plates would have a lower cross‐sectional load‐carrying capacity under fire.