A new in-situ metal matrix composite has been produced by liquid state reaction between aluminium and boron. The common boride phases obtained from this reaction are AlB12 and AlB2 of which the latter is stable at room temperature. One of the critical parameters of the reinforcement is the aspect ratio of AlB2 flakes. In this study, the Taguchi design method has been employed to examine the effects of three process parameters of additive elements to Al matrix, cooling rate in the AlB12(solid) + Al(liquid) region, and cooling regime in the AlB2(solid) + Al-(liquid) region on the size of AlB2 flakes. In the experiments, melting, casting, solidification, metallographic techniques, optical microscope, SEM and wet analysis techniques have been used. Results showed that 1.3 wt % boron has been dissolved in the aluminium at 1400 degrees C by adding boron oxide (B2O3) resulting in AlB2 flakes of 2.5 % by volume. It is also concluded that the AlB12(Solid) + Al-(liquid) region has to be passed with adequately high cooling rates in order to obtain AlB2 flakes with highest possible aspect ratios.