A Multiple-Input-Multiple-Output (MIMO) antenna device is adapted for connecting electrically to a radio frequency (RF) circuit for transmitting and receiving RF signals. The MIMO antenna device includes a circuit board, a plurality of antenna units, and a plurality of multiplexer units. The antenna units are disposed on the circuit board proximate to a peripheral edge thereof, are arranged in a loop formation, and are divided into a plurality of groups of the antenna units. Each of the multiplexer units is connected electrically to a respective one of the groups of the antenna units for selecting one of the corresponding antenna units and for connecting electrically the selected one of the corresponding antenna units to the RF circuit, thereby achieving the MIMO technique with the independently and simultaneously operating antenna units.