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Mg(BH4)2 was previously studied as a promising hydrogen storage material, because of its high gravimetric storage capacities for hydrogen and suitable thermodynamic properties. Mg(BH4)2 began to decompose at about 300 ℃, and formed MgB2 at 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(BH4)2 to MgB2 can be a potential method for fabricating superconducting MgB2 at a low sintering temperature. In this paper, MgB2 bulk was prepared by an in-situ reaction, using the Mg(BH4)2 pressed block as a precursor. The resistance change of the sample was monitored during the Mg(BH4)2 decomposition process and the resistance-temperature (R-T) curve of this process was recorded. Phase of MgH2, Mg and B were formed as the block slowly release its hydrogen before MgB2 occurred. According to the R-T curve, the phase formation of MgB2 started in a relatively low temperature of 410 ℃. Because MgB2 was critically formed by Mg and B derived from Mg(BH4)2, we can compare our formation temperature with previous study on MgB2 prepared by Mg and B in different particle size. The fitting result indicated that the particle size of Mg and B harvest from Mg(BH4)2 decomposition 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 MgB2 was 95.2%, and the size of MgB2 grains was 10–18 nm. SEM images showed that the MgB2 bulk had a porous structure and poor connectivity, which was caused by large amount the hydrogen release during the decomposition. MgB2 nanofibers 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 situ method to fabricate MgB2 showed 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 MgB2, which have the potential in industrial scale fabrication of MgB2 bulks and wires.