A fastening device used to fasten two locknuts to an object simultaneously includes a pedestal defining a first sliding groove, a pair of fixing blocks, a pair of rotating elements, a pair of first gear wheels and a second gear wheel. The fixing blocks are received and fixed in the first sliding groove. Each of the fixing blocks defines a through hole. Each of the rotating elements includes an operating portion and a shaft portion received in the through hole. Each of the pair of gear wheels includes a gear body attached to corresponding one of the shaft portions respectively and a sleeve protruding from the gear body and snugly receiving one of the locknuts. The second gear wheel is attached to the pedestal and engaging the pair of first gear wheels to achieve synchronous movements of the pair of first gear wheels.