Mg97Y2M1(M=Zn,　Ni,　Ag)　alloys　were　prepared　by　induction　melting　method.　The　characteristics　of　long　period　stacking　structures　(LPS)　formed　in　these　alloys　were　studied　by　TEM　and　SEM.　The　results　show　that　the　LPS　in　the　as-cast　Mg97Y2Zn1　can　be　described　with　a　6-layer　stacking　period,　which　is　different　from　the　typical　6H　type　with　a　hexagonal　structure.　The　existence　of　LPS　in　the　Mg-Y-Ni　system　has　also　been　found,　but　its　lattice　distortion　occurs　more　often　than　that　in　the　Mg-Y-Zn　system.　In　contrast,　no　LPS　structure　is　observed　in　the　Mg-Y-Ag　sysyem.　On　the　basis　of　experimental　results,　the　factors　affecting　the　formation　of　LPS　structures　in　Mg97Y2M1　alloys　are　also　discussed.