Mg(BH ₄) ₂was previously studied as a promising hydrogen storage material, because of its high gravimetric storage capacities for hydrogen and suitable thermodynamic properties. Mg(BH ₄) ₂began to decompose at about 300 ℃, and formed MgB ₂at the end of hydrogen desorption process with the weight content of 14.9% of hydrogen lost. Aside from the prominent hydrogen storage property, the decomposition process from Mg(BH ₄) ₂to MgB ₂can be a potential method for fabricating superconducting MgB ₂at a low sintering temperature. In this paper, MgB ₂bulk was prepared by an in-situreaction, using the Mg(BH ₄) ₂pressed block as a precursor. The resistance change of the sample was monitored during the Mg(BH ₄) ₂decomposition process and the resistance-temperature ( R- T) curve of this process was recorded. Phase of MgH ₂, Mg and B were formed as the block slowly release its hydrogen before MgB ₂occurred. According to the R- Tcurve, the phase formation of MgB ₂started in a relatively low temperature of 410 ℃. Because MgB ₂was critically formed by Mg and B derived from Mg(BH ₄) ₂, we can compare our formation temperature with previous study on MgB ₂prepared by Mg and B in different particle size. The fitting result indicated that the particle size of Mg and B harvest from Mg(BH ₄) ₂decomposition was only 3.4 nm on average. The nearly atomic level mixture of Mg and B resulted in a high chemical reactivity, which was the main reason for low sintering temperature. X-ray diffraction results showed that the purity of MgB ₂was 95.2%, and the size of MgB ₂grains was 10–18 nm. SEM images showed that the MgB ₂bulk had a porous structure and poor connectivity, which was caused by large amount the hydrogen release during the decomposition. MgB ₂nanofibers can also be observed inside the bulk. In the superconductivity test, the superconducting transition temperature of the bulk was 35 K. After all, such in situmethod to fabricate MgB ₂showed a great advantage in some aspects, as its low-cost precursors, low sintering temperature, small grain-size and high superconducting transition temperature in the formed MgB ₂, which have the potential in industrial scale fabrication of MgB ₂bulks and wires.