A case structure for an electronic device of the present invention at least includes a main body and at least one side piece detachably mounted to at least one side of the main body such that different sizes of case structures could be manufactured only by changing the size of the side piece while keeping the main body unchanged, thereby reducing molding cost for developing new electronic devices, saving a large amount of time needed to design, test and modify cases as well as saving human resources and reducing the manufacturing cost.