Using　the　base　line　forces　as　fundamental　variables,　the　complementary　energy　of　an　element　with　geometrical　nonlinearity　is　constructed,　which　contains　both　deformation　part　and　rotation　part.　Based　on　the　complementary　energy　principle　for　large　elastic　deformation　by　Gao,　the　equilibrium　conditions　are　released　by　the　Lagrange　multiplier　method,　and　a　modified　complementary　energy　principle　related　to　the　base　line　forces　is　obtained.　The　4-node　plane　element　model　with　geometrical　nonlinearity　is　derived　by　assuming　uniform　stress　distribution　on　each　faces　of　the　element.　A　nonlinear　finite　element　code　is　developed.　Numerical　results　show　that　this　model　works　very　well.