A 3-D imaging system based on sparse MIMO (Multiple Input and Multiple Output) array at 340 GHz is presented. The imager incorporates a linear array containing 4 transmitters and 16 receivers (4Tx-16Rx) as electronic beam former in horizontal dimension and an elliptic cylinder as focusing reflector in elevation to get high cross-range resolution. Each emitter transmits frequency-modulated continuous-wave (FMCW) waveform with 16 GHz bandwidth and high range resolution is achieved by using pulse compression technique. Experimental results indicate that the 3-D imaging resolution is 14 mm, 10mm and 12mm in horizontal direction, vertical direction and range dimension at 4-m distance, respectively. The personal screen experiments also imply that concealed threats such as guns can be detected by this 3-D imager.