The　theoretical　investigation　of　the　size　dependent　behavior　of　a　Bernoulli-Euler　dielectric　nanobeam　based　on　the　strain　gradient　elasticity　theory　is　presented　in　this　paper.　The　variational　principle　is　utilized　to　derive　the　governing　equations　and　boundary　conditions,　in　which　the　coupling　between　strain　and　electric　field,　strain　gradient　and　electric　field,　and　strain　gradient　and　strain　gradient　are　taken　into　account.　Different　from　the　classical　beam　theory,　the　size　dependent　behaviors　of　dielectric　nanobeams　can　be　described.　The　static　bending　problems　of　elastic,　pure　dielectric　(nonpiezoelectric),　and　piezoelectric　cantilever　beams　are　solved　to　show　the　effects　of　the　electric　field-strain　gradient　coupling　and　the　strain　gradient　elasticity.　Comparisons　between　the　classical　beam　theory　and　the　strain　gradient　beam　theory　are　given　in　this　study.　It　is　found　that　the　beam　deflection　predicted　by　the　strain　gradient　beam　theory　is　smaller　than　that　by　the　classical　beam　theory　when　the　beam　thickness　is　comparable　to　the　internal　length　scale　parameters　and　the　external　applied　voltage　obviously　affects　the　deflection　of　the　dielectric　and　piezoelectric　nanobeam.　The　presented　model　is　very　useful　for　understanding　the　electromechanical　coupling　in　nanoscale　dielectric　structures　and　is　very　helpful　for　designing　devices　based　on　cantilever　beams.