The phase transitions of the Al-B system were systematically explored by means of DSC and traditional solid-state reaction. Phase and microstructures were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively. The results show that the sintering temperature plays a decisive part in the decomposition of AlB2. At the same time, holding time also has an important influence on the decomposition of AlB2. AlB2 can decompose completely at 1050 oC for 3 h using the vacuum sintering method. The formation reaction of AlB12 is a complete reaction. The reaction between Al and AlB12 can’t take place regardless of stoichiometric Al/B heated in the range of 900 to 1150 oC for 3 h.