The　size-dependent　pull-in　voltage　of　electrostatically　actuated　MEMS　is　studied　using　strain　gradient　elasticity.　The　microbeam　model　and　microplate　model　are　derived　respectively　via　the　principle　of　minimum　potential　energy.　The　generalized　differential　quadrature　method　and　pseudo　arclength　algorithm　are　used　to　solve　the　high-order　PDEs.　It　is　shown　that　the　normalized　pull-in　voltage　predicted　by　the　new　models　increases　nonlinearly　with　the　decrease　of　the　structure　thickness,　exhibiting　size　effect　(and　the　size　effect　is　particularly　strong　when　the　structural　thickness　is　on　the　same　order　of　the　characteristic　material　length　scale　parameter);　while　the　corresponding　classical　models　do　not　exhibit　such　a　size　effect.　The　two　new　models　may　be　regarded　as　extensions　of　the　corresponding　classical　ones.　This　study　may　be　helpful　to　characterize　the　mechanical　properties　of　small　sized　MEMS,　or　guide　the　design　of　microstructures　for　a　wide　range　of　potential　applications.