An　investigation　was　carried　out　to　study　the　effects　of　aging　time　on　the　microstructure　and　mechanical　properties　of　A356　alloy　by　semi-solid　processing.　The　alloy　was　subjected　to　solution　treatment　for　4　h　at　540　degrees　C,　artificial　ageing　was　carried　out　at　temperature　(170　degrees　C)　and　different　time　(0-24　h).　The　results　revealed　that　the　needle-shaped　eutectic　Si　phase　in　as-cast　ingot　was　transformed　to　globular　shape　compared　with　that　by　solution　treatment.　Meanwhile　the　Chinese　script　pi-Fe(AlMgSiFe)　phase　was　transformed　to　smaller　and　uniformly　distributed　beta-Fe(AlFeSi)　phase,　the　elongation　increased　obviously.　With　the　increasing　of　aging　time,　the　elongation　decreased,　and　the　ultimate　tensile　strength　increased.　When　the　aging　time　was　4　h,　the　alloy　achieved　the　optimal　mechanical　properties.　The　micro-hardness　was　121HV,　ultimate　tensile　strength　was　329　MPa,　and　the　elongation　was　15%.　From　the　fracture　morphology,　the　fracture　for　alloy　aging　for　4　h　showed　more　dimples.　With　the　increasing　of　aging　time,　the　ultimate　tensile　strength　and　the　elongation　decreased.