The effect of chemically modified tips on STM imaging was investigated. An electrochemical technique was used to prepare a monolayer of sulfur and iodine on the apex of STM tips. The chemically modified STM tips were first employed to image a 1-octadecanethiol (1-C18H37SH) adlayer on a highly oriented pyrolytic graphite (HOPG) surface. The images acquired with S-modified tips could ascertain the structural detail of thiol groups of the adsorbed molecules, while that acquired with I-modified tips appeared in a low resolution. The contrast variations of STM images are attributed to the changes of electronic states of the tips.