We have studied the mechanism of the photoluminescence (PL) enhancement and quenching of single CdSe/ZnS core/shell nanocrystals on Au surfaces by means of single nanocrystal spectroscopy. The on-off PL blinking observed on the glass surface is drastically suppressed on Au surfaces, because of the fast energy transfer between Au surfaces and nanocrystals. The PL enhancement of single CdSe/ZnS nanocrystals occurs on rough Au surfaces, but PL quenching occurs on flat Au surfaces, compared to the case of the glass surface. Single nanocrystal spectroscopy reveals that the PL enhancement on rough Au surfaces is caused by the suppression of PL blinking and the electric field enhancement due to localized plasmon excitation.