A fastening device includes a first clamping arm, a second clamping arm pivotably connected to the first clamping arm and a mounting plate. The first clamping arm includes a first engaging portion. The first engaging portion defines a first threaded latching cutout. The second clamping arm includes a second engaging portion. The second engaging portion defines a second threaded latching cutout. The mounting plate attract the first engaging portion and the second engaging portion together. The first and second clamping arms are capable of being pivoted to make the first and second engaging portions abut against each other or moving away from each other. When the first and second engaging portions abut against each other, the first and second latching cutouts align with each other, to form a clamping hole to hold a threaded member.