Magnesium　matrix　composites　reinforced　with　AlN　particles　were　fabricated　by　the　powder　metallurgy　technique.　The　evolution　of　lattice　constants　and　solid　solubility　levels　of　Al　in　alpha-Mg　and　the　microstructure　of　Mg-Al/AlN　composites　were　investigated　in　the　present　study.　The　results　showed　that　the　solid　solubility　of　Al　in　a-Mg　reached　a　relatively　high　level　by　the　P/M　process　with　a　long　time　of　milling.　X-ray　diffraction　showed　that　the　peaks　of　Mg　phase　clearly　shifted　to　higher　angles.　The　lattice　constants　and　cell　volume　decreased　significantly　compared　with　those　of　standard　Mg　due　to　a　significant　amount　of　Al　incorporated　into　alpha-Mg　in　the　form　of　substitutional　solid　solution.　The　degree　of　lattice　deformation　decreased　at　a　low　sintering　temperature　and　increased　at　higher　sintering　temperatures　due　to　the　presence　of　AlN.　Microstructural　characterization　of　the　composites　revealed　a　necklace　distribution　of　AlN　particles　in　the　Mg　matrix.　Heat　treatment　led　to　precipitation　of　Mg17Al12　from　the　supersaturated　alpha-Mg　solid　solution.　The　precipitate　exhibited　granular　and　lath-shaped　morphologies　in　Mg　matrix　and　flocculent　precipitation　around　AlN　particles.