The adsorption of cytosine on metallic pristine single walled carbon nanotubes (SWNT) surface is investigated using density functional theory with local density approximation. On the SWNT, cytosine is physis orbed by taking the π-π interaction. Binding energy reported in this case is around -0.38 eV. By introducing metal atoms to the cytosine- SWNT, interaction can be strongly enhanced. The enhanced binding energies increases to -0.56 and -2.20 eV in presence of Li and Co atoms. Using pristine SWNT, electric sensor based on Co- doped SWNT depicts more sensitivity. Reported work gives insight into SWNT-based bio- sensors enhanced by doping appropriate metal atoms.