Kelvin probe force microscopy (KPFM) of DNA-single-walled carbon nanotube (SWNT) hybrids was performed for the first time. KPFM images indicated that the hybrids had a negative surface potential relative to a Au-Pd alloy surface. In contrast, the surface potential of SWNTs modified with poly(ethylene glycol) (PEG SWNTs) was positive even after they were wrapped by DNA. Modification with covalent bonding may have a significant effect on the potential. The results suggest a possible way to distinguish variously modified SWNTs through KPFM observation.