The duality solution for elasticity and the biorthogonality relationship have been well researched. Now the couple stress theory becomes a new research spot but there is few research for the biorthogonality relationship for couple stress theory comparing to classical elasticity. A new state vector is presented for three dimensional couple stress problems of prismatic structures. A new biorthogonality relationship of couple stress is discovered. The dual partial differential equations of couple stress problem are derived by the new state vector. By two important identical equations the new biorthogonality relationship is proved based on the method of separation of variables. The symplectic orthogonality relationship to three dimensional couple stress theory may be decomposed into two independently and symmetrically orthogonality relationships. The new biorthogonality relationship includes the symplectic orthogonality relationship. The biorthogonality relationship of couple stress may also be degenerated into the theory of elasticity. The new state vector and biorthogonality relationship provide theoretic foundation for the research on the schemes of separation of variables and eigenfunction expansion of couple stress theory.couple stress, duality solution, biorthogonality relationship, Hamiltonian, state spaceThe solution of elasticity is a classical problem which has lasted for more than a century. Because of the complication of its partial differential equations, the semi-inverse solution became the classical solution method in elasticity [1] . By the analogy theory between computational structural mechanics and optimal control, the Hamiltonian system theory was introduced into the theory of elasticity [2][3][4][5][6] . Based on the separation of variables of the Hamiltonian system, it was proved that among its eigenfunction-vectors state space there existed a symplectic orthogonality relationship and the corresponding expansion theorem. The duality solution of elasticity has reached a new stage [7][8][9][10][11][12][13][14][15][16][17][18][19][20] .The Saint-Venant problem and the Saint-Venant principle were unified by the direct method. It was proved that all the Saint-Venant solutions may be obtained directly via the zero eigenvalue solutions and all their Jordan normal forms of the corresponding Hamiltonian operator matrix and the Saint-Venant principle correspond to neglect of all the nonzero eigenvalue solutions, in which the non-zero eigenvalue gives the decay rate [21][22][23][24][25][26][27][28][29][30] . Recently, some researches on symplectic solution have drawn attention to the advantage of the state variable method [31][32][33][34][35][36][37][38][39][40][41][42] . It is noted that classical elasticity is well suited to the state space approach, including important biorthogonality relations. Vasil and Lur [43] presented a method of homogeneous solutions and biorthogonal expansions in the plane problem of the theory of elasticity. It was proved that the biorthogonality condition was equivalent to a g...