A fixing structure applicable to multiple sizes of storage device includes an enclosure, which has an interior forming an accommodation space that receives a storage device of one of the multiple sizes to be inserted into the enclosure. The enclosure includes, in an upper portion of the interior thereof, at least one spring-biasing section, which applies a spring force to an upper surface of the storage device opposite thereto for positioning in a longitudinal direction. The enclosure includes therein positioning sections and retention sections respectively set on opposite lateral sides thereof for cooperating with lateral sides of the storage device for positioning in a lateral direction. A lid has an inside surface on which a supporting section is formed so that when the lid is mounted to and closing a front side of the enclosure, the supporting section is in engagement with the storage device to secure the storage device.