A fastening structure is provided for fastening a thermal module to a mainboard, and includes a main body having at least one elastic press portion, a fastening portion, and an insertion unit. The fastening portion is provided on an end of the main body and the insertion unit is outward extended from another opposite end of the main body. The elastic press portion is provided on the main body and located between the insertion unit and the fastening portion, and a flexible space is defined between the elastic press portion and the main body. With these arrangements, the fastening structure can be quickly assembled to the thermal module without the need of welding and can therefore be conveniently separated therefrom whenever reworking is necessary.