Dramatic contrast variation of STM images has been investigated as a function of bias on a bis[1,3-di(p-n-tetradecyloxyphenyl)propane-1,3-dionato] copper(II) (abbreviated as C14O-Cu) adlayer physisorbed on HOPG with STM. It was found that STM images changed with bias change of 100 mV or less. Submolecular features of the molecular core and interdigitated alkyl chain were clearly revealed. A preliminary theoretical calculation was carried out to simulate the image variation. The simulated results were in agreement with STM observation.