Al2O3/Al composites were fabricated by replacement reaction of molten Al from kaolin instead of quartz glass as preform material. In this case the flexural strength of composites were deteriorated. In order to prevent the decrease in flexural strength by forming the $eta$-type alumina in situ reaction as reinforcement stone powder and clay were added to kaolin as alkali and alkaline earth sources respectively. Composites fabricated from either 2.5wt% stone powder or 5 wt%clay added kaolin preform showed the maximum value of 380MPa as determined by 3-point bending tests. When the content of alkali and alkaline earth in the preform exceed 2.7wt% the flexural strength of the composite decreased in spite of the increased amount of $eta$-type alumina