Mg-Ni-B system alloys were prepared by the mechanical alloying (MA) method. Body centered cubic (BCC) structure alloys are obtained in some of the Mg-Ni-B compositions after the starting mixtures of raw elements were ball milled for 200 h. Mg50Ni45B5 and Mg48Ni48B4 alloys after ball milling are with single BCC structure, which is confirmed by electron diffraction patterns. From the results of X-ray diffraction and transmission electron microscope, the crystallite size of the alloys is calculated into nanometer scale. Mg50Ni45B5 and Mg48Ni48B4 BCC alloys can absorb hydrogen at 373 K with higher rate than Mg50Ni50 alloy prepared in the same conditions. And these two samples can reach a hydrogen absorption capacity of 1.94 and 1.93 mass%, respectively at 373 K without any activation process.