Based　on　the　modified　couple　stress　theory　and　von　Kármán　nonlinear　theory,　a　size-dependent　nonlinear　mathematical　model　for　an　electrostatically　actuated　microbeam　with　a　piezoelectric　layer　bonded　to　the　top　surface　is　formulated　by　the　Hamilton’s　principle.　In　the　developed　model,　the　static　behavior　of　the　microbeam　is　discussed　by　using　numerical　methods.　The　results　show　that　the　size　effect　is　significant　when　the　microbeam　thickness　is　comparable　to　the　material　length　scale　parameter.　The　effect　of　geometric　nonlinearity　on　the　pull-in　voltage　mainly　depends　on　the　initial　gap.　By　applying　a　small　negative　voltage　to　the　piezoelectric　layer,　the　pull-in　voltage　may　effectively　reduce.　To　attain　accurate　analysis　for　the　static　behaviors　of　microscale　beam　devices,　the　damage　has　to　be　considered.　©　2017,　Springer-Verlag　Berlin　Heidelberg.