We have investigated constant-height atomic force microscope images of a hydrogen-terminated silicon (100) 2× 1 surface including a methyl observed by a silicon tip with and without a hydrogen atom at the apex, using a density-functional-based tight-binding method. Using silicon-tip without hydrogen at the apex, we obtain good images with anisotropic spots reflecting the symmetry of methyl when observing with large tip-sample distance. Using silicon-tip with hydrogen at the apex, we expect to obtain better images showing internal hydrogen and carbon atoms of methyl if the forces can be measured precisely: they have atomic resolution for the image of adsorbed methyl. [DOI: 10.1380/ejssnt.2006.197]