This paper aims to characterise the microstructure and the micro-mechanical properties of the interfacial transition zone (ITZ) in mortars at different water/cement ratios by nanoindentation and scanning electron microscope. Through this study, an insight is provided into the link between the microstructure and the micro-mechanical properties of the ITZ. The thickness of the ITZ was determined from the viewpoint of micro-mechanical properties, as well as from the viewpoint of porosity. The results showed that the average elastic modulus of calcium-silicate-hydrate gel present in the ITZ decreases with the increasing water/cement ratio. Meanwhile, the ratios between the average reduced moduli of the ITZ and the cement paste matrix were quantitatively determined, and the results showed that a lower water/cement ratio led to a thinner ITZ, and a higher ratio between the average reduced moduli of the ITZ and the cement paste matrix.