A server includes a chassis, a main tray and a hard disk drive (HDD). The main tray is slidably connected to the chassis and is capable of pulling out of the chassis or pushing into the chassis along a first direction. The main tray includes an accommodating slot and a sub-tray. The sub-tray is slidably disposed inside the accommodating groove and when the main tray is pulled out of the chassis, the sub-tray can be slid along a second direction perpendicular to the first direction between an expanded position and a retracted position relative to the chassis. When the sub-tray at the expanded position, the hardware can be taken from the sub-tray or put on the sub-tray.