The effect of reinforcement volume ratio on the infiltration behaviour of molten metal, thermal properties and microstructure of Al-MgO composite fabricated by vacuum infiltration method were investigated. Vacuum Infiltration is a composite production method in which reinforcement volume ratio is one of important parameters. In this study, MgO powder and Al were used as reinforcement and matrix, respectively. Mixture of MgO and Al powders with 105 mm particle size were filled in quartz tubes freely to form 10, 20 and 30% reinforcement volume ratio. Liquid Al was vacuum infiltrated into the MgO powder under 550 mmHg vacuum at 730 +/- 5 degrees C in normal atmosphere for 3 min Microstructure of composites. were investigated by SEM analysis. From these images, a thermal analysis model has been generated through which the effective thermal conductivity was determined. The effective thermal conductivity of Al-MgO composites increase as the reinforcement volume ratio decreases. The experimental and especially the numerical analysis results in this study gave thermal conductivity values which agree quite well with each other.