An understanding of the orthotropic plates' behavior in their dynamic regime is essential because the loading can cause severe damage in the plates, such as: - cracking, loss of aesthetic, fear to the occupants, etc. To this end, a new set of stress - strain relations for. Orthotropic plates were derived. The principle of force of inertia was introduced, yielding the corresponding dynamic governing equation of orthotropic plate. The solution of the equation was obtained by numerical method, and the results show that the flexural rigidity of the bars has significant effect on the fundamental natural frequency of heavily reinforced concrete sections.