Abstract: The effects of SSC(small-scale sub-lithospheric convection) on seafloor topography are investigated by formulating a 2D thermal convection model that is parallel to plate motion. Instead of closed boundary conditions which will bring large pressure effects because of return flow, a flow through boundary condition is adopted. The results show that the SSC can cause the topography flattening only in constant viscosity model; for the model with temperature-dependent viscosity, although the SSC enhances the surface heat flux, it has little effects on topography.