Summary: The strain gradient theory of S. Zhou et al. [“A reformulation of constitutive relations in the strain gradient elasticity theory for isotropic materials”, Int. J. Solids Struct. 80, 28–37 (2016; doi:10.1016/j.ijsolstr.2015.10.018)] is re-expressed in a more direct form and the differences with other strain gradient theories are investigated by an application on static and dynamic analyses of FGM circular micro-plate. To facilitate the modeling, the strain gradient theory of Zhou et al. [loc. cit.] is re-expressed in cylindrical coordinates, and then the governing equation, boundary conditions and initial condition for circular plate are derived with the help of the Hamilton’s principle. The present model can degenerate into other models based on the strain gradient theory of D. C. C. Lam et al. [J. Mech. Phys. Solids 51, No. 8, 1477–1508 (2003; Zbl 1077.74517)], the couple stress theory, the modified couple stress theory or even the classical theory, respectively. The static bending and free vibration problems of a simply supported circular plate are solved. The results indicate that the consideration of strain gradients results in an increase in plate stiffness, and leads to a reduction of deflection and an increase in natural frequency. Compared with the reduced models, the present model can predict a stronger size effect since the contribution from all strain gradient components is considered, and the differences of results from all these models are diminishing when the plate thickness is far greater than the material length-sale parameter.