A casing structure with functionality of effective thermal management is disclosed, which consists of a casing member, a low thermal conductivity medium, a second heat spreader, and a first heat spreader. When a user operates the electronic device, heat generated from CPU and/or GPU is transferred to the second heat spreader via the first heat spreader, and then is two-dimensionally spread in the second heat spreader. Consequently, the heat is dissipated away from the casing member to air due to the outstanding thermal radiation ability of the casing member. The low thermal conductivity medium is adopted for controlling a heat transfer of heat transferring paths from the heat source and ends to the casing member. By applying the casing structure in an electronic device by a form of a top casing and/or a back casing, an outer surface temperature of the casing member can be well controlled.