The microstructure and the mechanical properties at room and high temperatures of monocrystalline Ni3Al containing 0, 0.52, 1.37, 2.22 and 3.00 at%B have been studied. The results show that the ultimate and yield strengths at 20°C and 800° C of monocrystalline Ni3Al obviously increase with the increase of B content, and reach maxima when B content is 1.37 and 0.52 at% respectively. With further increasing the B content to 2.22 at%, they slowly drop. The tensile ductilities at 20°C and 800°C of monocrystalline Ni3Al when the B content is 0 and 0.52 at% respectively are the best in five kinds of Ni3Al studied, then they continuously decrease with the increase of B content due to the precipitation of boride eutectic. The rupture life and elongation at 850°C and 100 MPa markedly drop as the B content is raised in the monocrystalline Ni3Al, but they are still higher by 1 to 2 orders of magnitude as compared with those of polycrystalline Ni3Al with the same compositions. When B content exceeds 1.37 at%, the eutectic structure of Ni20Al3B6 and Ni3Al is formed, which is enhanced by the B content and is harmful to the strength and the ductility.