Compared with homogeneous and isotropic structures, FGM structures possess so-called stretching-bending coupling effects, which mainly caused by the continuous and smoothly varying of constituent in some specific directions.Many works showed that the inplane force resultants and the out-plane moments of FGM structures have an impact on each other and are irreducible in the constitutive equations.So it seems that the stretching-bending coupling is an inherent character of FGM structures.However, Zhang and Zhou (D.Zhang and Y.Zhou, "A theoretical analysis of FGM thin plates based on physical neutral surface", Comp.Mater.Sci., 44, 716-720 (2008)) eliminated the stretching-bending coupling effects in the constitutive equations in both small and large deflection problems of functionally graded (FG) thin plates under a proposition of the physical neutral surface.This work presents a theoretical analysis of bending FG circular thin plates, and tries to gain an insight into the stretching-bending coupling effects.Under the assumption that Poissons ratio is constant and independent of thickness coordinates, we give a comparative discussion of neutral-surface based formulation and middle-surface based formulation for bending of FG circular thin plates in both small and large deflection situations.We reveal an interesting result that both neutral-surface based formulation and middle-surface based formulation will lead to completely identical theoretical model without so-called stretching-bending coupling.Then we show that stretching-bending coupling effects of FG plates result mainly from boundary conditions.As illustrative examples to verify the influence of boundary conditions on stretching-bending coupling of FGM circular thin plates, we give a discussion about axisymmetric bending of FG circular thin plates under different boundary conditions, including uniformly distributed lateral load with clamped and simply support, in-plane compression and combined action of lateral load and in-plane compression.The in-plane restrictions acting at middle surface and neutral surface are all considered.