The uniaxial tensile and compression tests of the Cu∑ 5 grain boundary (GB) with and without segregated interstitial boron have been performed using first principles method based on density functional theory. Results show that boron enhances the cohesion of Cu∑5 GB and improves the mechanical property of Cu significantly. The clean boundary has lower density of valence electrons than perfect lattices and will be the point for fracture to start under sufficiently high tensile stress. The Cu∑5 GB with segregated boron has strengthened the cohesion across the boundary because of the strong B-Cu bond. Charge accumulated to Cu-B decreases slightly the strength of neighboring Cu-Cu bonds, which will be the weak point for fracture to initiate. The ultimate tensile stress is enlarged by the addition of boron. There is no significant effects occurring within 20% of the compression strain due to B-doping.