An assembly structure provided for securing a heat-dissipating fan, includes a casing board and a fan covering for covering the fan, The casing board has ventilation openings, coupling openings and positioning openings, and the fan covering formed with coupling portions and positioning portions secures the fan to a side of the casing board, wherein the coupling portions are coupled to the coupling openings, and the positioning portions are positioned and coupled to the positioning openings, so that the fan is installed at a location allowing wind or airflow to pass through the ventilation openings. Arrangement as such, without using tools during assembling or dissembling processes, allows two heat-dissipating fans to be correspondingly coupled at both sides of the casing board, so as to increase wind force to increase heat dissipation in a simple but convenient way.