The authors think that there exist two kinds of size effects for the stability of surrounding rock mass of an underground opening; namely they are the constitutive size effect and the structural size effect. As a tentative inquiry

the authors try to investigate the actions of the two kinds of size effects in the finite element analysis (FEA) for an underground opening model. So two cases are assumed: the first one

the mechanical properties at any point in surrounding rock mass are the negative power functions of distance between the point and the opening center; and the opening span changes from small to large

that is

the constitutive size effect and the structural size effects are considered simultaneously; the second one

the mechanical properties of surrounding rock mass are constants

but the opening span changes from small to large; that is

only the structural size effect is considered. The elastoplastic calculations under a plane-strain condition are carried out for the two cases; and the stresses

displacements and plastic zones in the surrounding rock mass are analyzed. The results show that: the displacements and plastic zones in the the surrounding rock mass are smaller; and the stresses are comparatively large

considering the constitutive size effect of the mechanical properties; but the situation of without considering the constitutive size effect is the other way round.