A fan module and a fan rail thereof are provided. The fan module includes a fan, two fan rails, and a holder. The fan rails are disposed at both sides of the fan, in which each fan rail includes a main body and a ring, the main body has a first end and a sliding slot. The sliding slot is located at the first end. Each ring has a tab portion and a connecting portion, wherein the connecting portion has a protrusion buckled in the sliding slot. The fan is disposed in the holder by means of the fan rails, and a force is applied on the tab portion to make the protrusion slip in the sliding slot so that the ring suits to move back and forth on a longitudinal direction of the main body relatively to the main body so as to detach the fan from the holder.