A data storage device, a stacking method thereof, and a data storage device assembly are provided. The data storage device assembly includes a first data storage device and a second data storage device respectively having a body, a magnetic element, and a storage device. Each body has a first containing space and a second containing space. Each magnetic element is disposed in the corresponding first containing space. At least one of the magnetic elements of the first and the second data storage device is a magnet. Each storage device is disposed in the corresponding second containing space and includes an electrical connector terminal, a memory chip, and a memory controller with no crystal oscillator. The magnetic elements of the first and the second data storage device attract each other so that the body of the first data storage device is stacked on the body of the second data storage device.