The present invention provides a case assembly structure of an electronic device. The case assembly structure includes a first case, a second case and a heat-dissipating element. The first case has an engaging hole and a first opening. The second case has an engaging element engaged with the engaging hole of the first case and a second opening aligned with the first opening of the first case. The heat-dissipating element includes a fastening hole aligned with the first opening of the first case and the second opening of the second case. The first case, the second case and the heat-dissipating element are combined together by penetrating a fastening element through the first opening, the second opening and the fastening hole.