A fastening device including a case unit, a knob and an engaging unit is provided. The case unit includes a wall forming a receiving space and a plurality of mounting teeth. The knob covers on the case unit. The engaging unit is within the receiving space. The engaging unit corresponds to the mounting teeth and is selectively coupled to the knob. The knob is switched between a first position and a second position. When the knob is in the first position to couple to the engaging unit, the engaging unit is linked up with the knob and the engaging unit is engaged with the mounting teeth to prohibit the knob from rotating in a first direction, and when the knob is in the second position to disengage from the engaging unit, the engaging unit does not prohibit the knob from rotating in the first direction.