With two-dimensional synthetic apertures,a wideband radar array can realize 3D imaging of air targets,but this needs large numbers of antenna elements.Furthermore,when the transmitting frequency is very high,the interval of array elements makes it hard to fit the available space,which makes the imaging resolution worse.This paper proposes a type of M 2 -transmitter N 2 -receiver multiple-input multiple-output (MIMO) radar,which is equivalent to a rectangular radar array with (M N) 2 T/R antenna elements based on phase center approximation theory,and which can realize the 3D imaging of air targets.Corresponding 3D imaging theory and MIMO inverse synthetic aperture radar (MIMO-ISAR) 3D imaging theory are also proposed in this paper,and two simulations are given to show their validity. With two-dimensional synthetic apertures, a wideband radar array can realize 3D imaging of air targets, but this needs large numbers of antenna elements. Future, when the transmitting frequency is very high, the interval of array elements makes it hard to fit the available space, which makes the imaging resolution worse. This paper proposes a type of M 2 -transmitter N 2 -receiver multiple-input multiple-output (MIMO) radar, which is equivalent to a rectangular radar array with (MN) 2 T / R antenna elements based on phase center approximation theory, and which can realize the 3D imaging of air targets. Corresponding 3D imaging theory and MIMO inverse synthetic aperture radar (MIMO-ISAR) 3D imaging theory are also proposed in this paper, and two simulations are given to show their validity.