Abstract: In last decades functionally graded materials become very popular for lots of industries such as automotive, naval, railroad, aerospace, etc. There are many papers in literature for bending, stability and vibration analysis of functionally graded plates. In this paper, bending of simply supported functionally graded square plates are studied. For functionally graded plates many shear deformation theory is offered. Various shape functions for defining displacement fields are used in these theories. In this study, several shape functions are discussed for deflection and stress distribution of functionally graded plate with sinusoidal loading. The exponential gradient form is assumed for change of material properties through thickness direction. Refined plate theory with different shape functions is used. Governing equations are derived from the principle of virtual displacements. The solution is obtained by Navier’s double trigonometric series approach. Numerical results of deflection, normal stress and shear stress are presented for thin and thick square plates.