Intermolecular attractive forces lead to the adhesion problem in M/NEMS. The Van der Waals formula for the interaction between macroscopic objects can be used only in the situation with no deformation. As to the adhesive contact between elastic bodies it is still unknown how the attractive force contributes to the normal force on the interface. In this paper large-scale molecular dynamics simulation is performed to study the adhesive contact between a rigid spherical tip and an elastic flat substrate. We study the effect of atomic-scale surface roughness on the adhesive properties, including pull-off force between tips and substrate, the variation of adhesive force with applied load, and the distribution of contact stress. The results show that the adhesive force varies linearly with the applied load for the atomic-scale smooth contact. But for the atomic-scale rough contact the variation of adhesive force with applied load can be divided into two phases, which are distinguished by different increasing slops. Compared with the smooth contact, the rough contact has a small pull-off force, but exhibits a large adhesive force during the contacting process. Our simulations indicate that the pull-off force cannot characterize the contribution of attractive interaction to the normal force on the interface in the case of an elastic adhesion contacting.