Abstract: A cap for fastening devices comprises an inner cap member fittable over a hexagonal engageable portion of the fastening device and having a top wall centrally formed with a threaded bore, an outer cap member fittable over the inner cap member and having a bolt hole formed in a top wall thereof and corresponding to the threaded bore in position, and a bolt to be screwed into the bore through the bolt hole. The inner cap member has a peripheral wall formed with six deformation promoting slits extending longitudinally thereof. The outer surface rear end portion of the peripheral wall has projecting tapered portions for inwardly moving the rear end portion of the inner cap member when the outer cap member is slidingly moved relative to the inner cap member by rotating the bolt relative to the inner cap member in a screwing-in direction.