A low-current scanning tunneling microscope (STM) was used to image ordered alkanethiol self-assembled monolavers (SAMs) on Au (111) surface. The effects of the tunneling current on the STM images of both decanethiol and dodecanethiol SAMs were studied. At low current, with the tip above the sample surface, domain boundaries and ordered alkanethiol molecules can be seen clearly. When the tunneling current was increased, destructive STM images of decanethiol and dodecanethiol SAMs were observed. However, when the tunneling current was reduced back to the original low values, the ordered structures of alkanethiol molecules reappeared. This reversible process suggests that there is no rearrangement in the sulfur head groups under different tunneling current conditions. In this article, we present a model which supports the important role of the alkyl chain in the STM imaging of SAMs. In our model, the medium conductance is a key factor.