By means of the three-dimensional (3D) elastic theory, a state equation is deduced for the exact analysis of laminated orthotropic plates under a harmonic temperature variation T = T′ (x,y,z). The exact solution for the response of simply supported rectangular laminated orthotropic plates to applied heating is given by solving the state equation. Finally, the numeric results of boundary surface stresses of every ply of laminated orthotropic plates are given by two numeric examples.