Aluminium Silicon reinforced with 50Vf% SiC has been produced by spontaneous infiltration at 900°C for 1 hour. Aluminium infiltrated preforms containing 1%wt Mg mixed with various of Si between 2 and 14wt%, as external dopant. However Al did not infiltrate a preform containing 1wt%Mg but if mixed with Si in the preform generated in more extensive infiltration. Effect of Si on characterisation of pure Al composites by spontaneous infiltration were studied and compared to Al-Si based matrix. Microstructural analysis of MMC as well as mechanical properties were also observed. It was found that increasing of Si content generated lower porosity thus increasing hardness due to aluminium could wet SiC preform well. Although the dopant was uniformly distributed throughout the perform but microstructural analysis and hardness measurements indicate that the resultant composite may not be uniform as infiltration inwards from the top to the bottom of preform. The hardness of Al-Si composites is significantly increased with increasing of Si for both externally and internally doped system. This is associated with decreasing porosity with higher Si in composites.