A storage system adapted for receiving HDDs of different dimensions, including a casing, a backplane, at least one power supply module, and at least one storage controller is disclosed. The internal space of the casing forms at least one HDD receiving chamber. The HDD receiving chamber can be used for selectively receiving first type HDD devices or second type HDD devices. The two types of HDD devices are of different dimensions, for example, 2.5″ and 3.5″ HDDs. The backplane, the power supply module, and the storage controller are provided in the internal space of the casing. The backplane has a plurality of first connectors for electrically connecting to the HDD devices. The storage controller is electrically connected to the backplane and power supply module. A storage system capable of receiving mixed HDD devices of different dimensions is formed thereby.