A heat dissipating device includes a fin unit disposed on a supporting seat, and an anchoring frame disposed on the fin unit and engaging detachably the supporting seat for anchoring the fin unit between the supporting seat and the anchoring frame. A fan module includes a fan unit having a fan blade disposed in a fan housing that is mounted on the fin unit, a connecting seat mounted on the fan housing and formed with a through hole that is in fluid communication with an inlet hole in the fan housing, and an air guiding member mounted on the connecting seat and confining an air passage that is in fluid communication with the through hole in the connecting seat. The fan unit is operable so as to draw external air toward the fin unit via the air passage.