Abstract: The micro-mechanical properties at grain boundaries of interstitial-free steel (IF steel) with different grain size were characterized by nanoindentation technique, and the Vickers hardness and tensile properties of the steel were tested. The effect of micro-mechanical properties at the grain boundaries on the macro-mechanical properties was discussed. The results show that the nanoindentation hardness and elastic modulus of the grains with different size were basically the same. The average nanoindentation hardness of the sample containing relatively fine grains was about 3.12 GPa, higher than that (2.36 GPa) with relatively coarse grains, and the elastic modulus was about 205 GPa, lower than that (210 GPa) with relatively coarse grains. The Vickers hardness and tensile strength of the sample containing relatively fine grains were also higher than those containing relatively coarse grains. The nanoindentation hardness at the grain boundaries (above 3.25 GPa) was higher than that (2.61 GPa) in the grains while the elastic modulus was slightly lower, which was the main reason for the difference between the macro-mechanical properties.