A new antenna structure with a pyramidal horn and a dielectric lens was investigated for a long-range automotive radar in the 77-GHz band. The use of a dual pyramidal horn and dielectric lens antenna reduces the height of the antenna while keeping the antenna gain high. The fabricated antenna was low in height and had an antenna efficiency of 45%, which is twice that of a conventional series-fed microstrip patch array antenna with almost the same aperture size. The antenna had a flat beam whose 3-dB beamwidth was 4.5 degrees in elevation and 15 degrees in azimuth, respectively, meaning that the antenna can be used for long range radar with a single beam.