In this study, a new functional is obtained for folded plates with geometric (kinematic) and dynamic (natural) boundary conditions. This functional is the combination of two different functionals. Both functionals are obtained for thick plates which carry in-plane and lateral forces. A new mixed finite element is developed with $4{ imes}13$ nodal parameters for folded plates (REC52). Forces and moments which are the necessary unknowns in engineering problems are obtained directly using the technique suggested here. The use of the global co-ordinate system causes time consuming operations and therefore the Lagrange multiplier method is used to relate the components of the parameters on the fold line. Numerical results are presented for folded plates and compared with experimental results.