Temperature dependence of blinking in SERS (surface enhanced raman scattering) signal, which is abrupt repeated intensity and peak frequency fluctuation, was studied. It was found that the blinking observed at room temperature (RT) was suppressed by cooling to 77 K for one-third to one-fourth of the particles and recovered by warming to RT. Accordingly, the blinking is thermally activated, e.g., by diffusion of molecules between different sites on Ag particles. These sites are possibly attributed to junctions of touching particles with enormously large electric fields and other ordinary sites with modest enhancement, as predicted by numerical simulations of the local electric field. The thermal activation is compatible with photochemical activation for particular molecules with appropriate electronic states for efficient charge transfer against metal surfaces.