An in situ Al-TiB2 nanocomposite was synthesized by mechanical alloying (MA) of pure Ti, B and Al powder mixture in a planetary ball mill. A double-step process was used to prevent the formation of undesirable phases like Al3Ti intermetallic compound. In the first step, a powder mixture was tailored to obtain nominal Al-90 wt% TiB2 composition and the second step involved the addition of Al to the mixture in order to achieve Al-20 wt% TiB2. The structural and thermal characteristics of powder particles were studied by X-ray diffractometry (XRD), scanning electron microscopy (SEM), differential scanning calorimetery (DSC), and transmission electron microscopy (TEM). The results showed that the MA process leads to the in situ formation of nanosized TiB2 particles in an Al matrix with a uniform distribution. It was also found that the double stage addition of aluminum can prevent the formation of undesirable compounds even after annealing at high temperatures(.).