Aluminum borate (Al18B4O33) whisker reinforced AZ91D and ZK60 magnesium alloy matrix composites were prepared by comp-casting process. Some composites were hot extruded after the casting. The bending strength at room temperature and 473K and age hardening behavior were estimated. As the interfacial reaction between Al18B4O33 and matrix in AZ91D matrix composites are very little during casting and aging, these composites have good mechanical properties and heat stability. Furthermore hot-extrusion are very effective for improvement of the bending strength because of the refinement. of the microstructure and degradation of the porosity. However, ZK60 matrix composites have low mechanical properties because of some interfacial reaction during preparation process.