Highlights • Invariant definition of the transversely isotropic constitutive tensor based on representation theorem of functionally graded materials (FGM) in oblique coordinates. • Study of influence of fiber direction and skew angle on natural frequencies of (FGM) nanocomposite skew plates. • Consideration of one asymmetric and three symmetric distributions of functionally graded nanocomposite skew plates. • The underlying shell theory is formulated in oblique coordinates, based on the Hu-Washizu principle and first-order shear deformation theory (FSDT). • Independent approximations of displacements, strains and stresses.