Automotive radar is an important environment perception sensor for advance driving assistance system. It can detect objects around the vehicle with high accuracy and it works in all bad weathers. For traditional automotive radar, it cannot measure the objects’ height. Thus, a manhole cover on the road surface or a guideboard high above the road would be taken erroneously as a non-moving car. In such cases, the adaptive cruise system would decelerate or stop the vehicle erroneously and make the driver uncomfortable. A 3D automotive radar with two-dimensional electronic scanning can measure the targets’ height as well as the targets’ azimuth angle. This paper presents a 79GHz ultra-wide band automotive 3D imaging radar. Due to the 4 GHz wide bandwidth, the range resolution of this radar can be as small as 3.75 cm. With the help of high range resolution and speed resolution, the three-dimensional appearance of the target vehicles, guardrail, manhole cover can be well depicted by the output point cloud. Thus, the performance of imaging the environment of the millimeter-wave radar is comparable to lidar. As the output of the automotive 3D imaging radar is similar to a 3D photo, sensor fusion of radar and camera can be more efficiently implemented. The radar imaging performance of several different traffic scenarios is modeled in a radar system simulator built by SystemVue and MATLAB. This radar can depict the appearance of the car, truck and guideboard with high accuracy.