This study presents an advanced method to estimate mechanical properties of structural steels through the instrumented indentation techniques using spherical indenter. Finite element analysis model of instrumented indentation testing was constructed to investigate the effect of stress.strain curve of materials on the indentation load.depth curve and the indentation contact radius. From calculation results, new estimation formulae for the representative stress and stain relations were established. Meanwhile, the applicable range of plastic constraint factor for completely.plastic region was quantified according to the indentation geometry and mechanical properties of materials. The newly.developed estimation method with the instrumented spherical indentation techniques was applied to various structural steels for its accuracy evaluation. It is then concluded that the developed method is useful to accurately estimate the mechanical properties of structural steels.