A new design and experimental results of a microstrip-fed ultra-wideband Fermi antenna at millimeter-wave frequencies are presented. By utilizing a new microstrip-to-CPS balun (or transition), which provides wider bandwidth than conventional planar balun, the design of microstrip-fed Fermi antenna is greatly simplified. The proposed Fermi antenna demonstrates ultra-wideband performance for the frequency range of 23 to over 58 GHz with the antenna gain of 12 to 14 dBi and low sidelobe levels. This design yields highly effective solutions to various millimeter-wave phased-arrays and imaging systems.