In this work, the effects of chromium addition on the morphology and mechanical properties of Fe2B in Fe-3.0B alloy have been systematically investigated by a combination of experimental observations and the first-principles calculations. The results indicate that, with chromium addition in Fe-3.0B alloy ranging from 0 to 2.5 wt%, the average grain size of the boride firstly decreases and then increases slightly, mainly attributed to the volume fraction of coarse block boride. The nanoindentation hardness H of the boride remains almost constant while the elastic modulus E-r firstly drops and then rises. Accordingly, the H/E-r of the boride achieves a peak value at the chromium addition of 2.0 wt%, where the highest toughness is obtained. High-resolution transmission electron microscopy (HRTEM) observation demonstrates that the lattice of borides evolves from tetragonal to orthorhombic ((Fe, Cr)(2)B) after chromium addition, in good accordance with the calculation results. During the lattice evolution, a shrinkage of B-B bond along [002] direction is simultaneously revealed. The inherently weak B-B bond can be strengthened, which improves fracture toughness of Fe2B.