A heat dissipating device comprises a dust-collecting mechanism installed at the air vent passage of a fan, and thus the dust or particles mixed in the airflow of the fan shall be collected on the top of the dust-collecting mechanism; the dust-collecting mechanism can be taken out frequently to remove and clean the collected dust or particles, so that the airflow of the fan can be kept normal and the dust deposited on the surface of the metal fins of a heat sink can be decreased greatly as to normally maximize the heat dissipating efficiency for the heat dissipating device for a long period of time and maintain the heat generating source to operate normally at a permissible temperature.