The　phenomenon　of　Portevin-Le　Chatelier　(PLC)　of　AZ91D　pressed　magnesium　alloy　with　different　contents　of　precipitated　phase　was　studied　during　tensile　processes,　and　the　influence　of　precipitates　on　PLC　effect　as　well　as　the　corresponding　mechanism　of　PLC　effect　were　studied　in　detail.　The　results　show　that　the　main　factor　for　the　PLC　effect　is　the　concentration　of　Al　solute　atoms　that　dissolved　into　the　magnesium　matrix,　and　the　β-Mg17Al12　precipitation　restrains　the　plastic　instability　behavior　of　AZ91D　alloy.　The　all　corresponding　stress-strain　curves　are　smooth　when　the　content　of　β-Mg17Al12　phase　is　high,　but　the　stress-strain　curves　show　apparently　serrated　yielding　and　more　serious　with　the　precipitation　of　β-Mg17Al12　phase　decrease　and　the　Al　atoms　dissolved　in　magnesium　matrix　increase.　Only　one　saw-tooth　appears　in　the　corresponding　stress-strain　curve　when　the　amount　of　β-Mg17Al12　precipitates　is　1.0　mass%,　indicates　that　the　critical　content　of　precipitated　phase　for　the　PLC　effect　of　the　AZ91D　alloy.　©　2016,　Editorial　Office　of　Transactions　of　Materials　and　Heat　Treatment.　All　right　reserved.