Based on the nitridation reaction of aluminum with boron nitride (BN), aluminum nitride (AlN) matrix composites were fabricated by reaction synthesis technique. The effect of the amount of Al on the microstructure and properties was investigated. The bending strength, thermal conductivity and dielectric constant increase with the content of aluminum. A heat-treatment schedule was performed to investigate the effect of the microstructure on the properties. The results showed that the heat-treatment leads to the grain growth and thermal conductivity increase with the grain growth.