The current-voltage (I-V) characteristics of organic molecules anchored covalently to silicon surfaces were studied with a scanning tunneling microscope in ultrahigh vacuum. It was found that molecules on silicon were not stable in the I-V measurement. Molecules were removed during the measurement, which was possibly induced by charge injection. Artificial negative differential resistance was sometimes recorded in the I-V plot. The origin of this event was explained by deformation and/or desorption of molecules induced by the charge injection during observation.