Two types of ultra-wideband tapered slot antennas (TSAs) are designed at millimeter-wave frequencies using the new microstrip-to-CPS transition (balun). Since the antenna feed structure including the balun provides wider bandwidth than TSA antennas, the overall bandwidth of the antenna is not noticeably changed with the feed structure. The designed antennas (LTSA and ETSA) show ultra-wideband performance from 23 to over 80 GHz. The antenna gain of LTSA ranges from 7 to 12 dB, and the front-to-back ratio from 12 to 20 dB. The antenna gain of ETSA ranges from 8 to 10 dB, and the front-to-back ratio from 14 to 30 dB.