Abstract: Synthetic aperture radar can obtain two-dimensional high resolution radar images, but it has to work in side-looking mode without forward-looking imaging capability, and can not focus stationary and moving targets simultaneously as well. Virtual aperture on the multiple-input multiple-output theory is an efficient solution to the aforementioned new problems of radar imaging. In this paper, antenna configuration and associated image formation of virtual aperture imaging are studied. Linear virtual apertures of different antenna configurations are compared in terms of imaging performance. It is theoretically proved that two transmitting elements are placed on the ends of received array is an efficient antenna configuration scheme to form a linear virtual aperture. The analytical expression of imaging resolution of this antenna configuration is derived, which is also validated with real data.