Functionalization of graphene with active functional groups such as thiol groups is higly desirable for application in highly sensitive graphene-based devices including chamical and biosensors. Herein, we report a powerful and common chemical method for the functionalization of graphene with varieties of active functional groups. Instead of using presynthesized stable diazonium salts, the reactant amine is used for graphene functionalization, which gives much more flexibility for functionalizing graphene with desired end groups. Using two different molecules, namely, 2-aminoethanethiol (HSC2H4NH2) and 3,5-difluorophenylamine (F2C6H3NH2), we found that amine-derived diazonium salts undergo spontaneous in situ reduction on graphene preloaded in the reaction tube, resulting in graphene functionalized with the organically active functional species HSC2H4 and F2C6H3. Pristine and modified epitaxial graphene (EG) on SiC were characterized using X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and scanning tunneling microscopy/spectroscopy (STM/STS) at room temperature. The present functionalization method can be utilized to functionalize graphene with varieties of active groups because a huge number of functional amines are commercially available.