In the recent years three-dimensional (3D) elastic-plastic analyses have been conducted extensively for the opening mode (mode I) fracture and the constraint effects are discussed in detail. However less work is focused on other modes as sliding mode (mode II), tearing mode (mode III) or the mixed mode fracture in three-dimensional. In this paper the thickness effect on pure mode II case is discussed by the finite element method (FEM). Modified Boundary Layer (MBL) model is used, which has the ability to take into account the combined effects of the in-plane constraint (T-stress) and the out-of-plane constraint (finite thickness). The result demonstrates the weak thickness dependence on the near tip stress and strain fields under mode II loading. And the size of the 3D zone at mode II loading is determined to range from 1.0 to 1.2 times the thickness. Two fracture parameters of J integral and crack tip sliding displacement (CTSD) are discussed, which are almost same at different thickness planes except those very near the surface. It is interesting to find that the relations between J and CTSD keep linear at different thickness planes. T-stress is symmetry on stress and strain distributions along the crack plane. However its effects indicate weak thickness dependent on the CTSD and J integral fracture parameter.