For optical diagnosis of the human body, using near infrared (NIR) has several advantages: NIR penetrates into the tissue deeper than UV or visible light, and in NIR most of the tissue-originated fluorescence may be avoided. Although NIR fluorophores are valuable, only a few can be used for humans and they have relatively low quantum yields. If the fluorescence emission of NIR fluorophores can be artificially enhanced, it can increase the sensitivity of optical diagnosis. In addition, conditionally emitted contrast agents as in Förster resonance energy transfer (FRET) or molecular beacon can be developed. One way of artificially changing fluorescence is by applying an electric field to the fluorophore. An excellent way of generating the field is via the plasmon field by gold nanoparticles (GNPs) upon the receipt of the excitation light of the fluorophore to be used. In this paper, the mechanism of the fluorescence manipulation for an NIR fluorophore, Cypate, by GNPs is studied both theoretically and experimentally.