Abstract: The effective torsional rigidity is equal to the geometricmean of two principal bending rigidities, as is characteristic of the orthotropicplates with smooth up and down surfaces. According to thatcharacteristic andbased on the differential equation of the deformed surface of theorthotropic plate, with one principal direction size fixed,changing another principal direction size linearly, maintainingelasticity coefficient the same as in the first principal direction andPoisson ratio equal to the geometric mean of two principal Poisson ratios, theoriginal orthogonal plate is equivalent to an isotropic plate. Resultsobtained show that the deflections of the equivalent isotropic plateare the same as the deflections of the original orthotropic plate at thecorresponding points, the forces of the correspondent points have simplerelations. The method of the paper will be easy to use for engineers andtechnicians.