Based on the rectangular micro-coaxial fabrication technology, a surface micro-machined 10 GHz to 50 GHz ultra-wideband planar log-periodic antenna is designed under the requirement of lightweight for the wideband electronic system in this paper. To match the nominal antenna impedance of 160 Ω to 50 Ω, a three-step impedance transformer is designed. Besides, a rectangular micro-coaxial line to CPW transition is used to feed the antenna. To display the advantages of the micro-coaxial feeding line and demonstrate the performances of the designed antenna, some 50 Ω micro-coaxial lines and the above log-periodic antenna have been fabricated and measured. The measured results show that, through the whole frequency band, the micro-coax lines have insertion loss as low as 0.22 dB/cm, and high isolation of 60 dB for the neighboring coaxial lines at a distance of 0.15 mm. The designed antenna has return loss as low as -8 dB, and antenna gain above 4 dBi with gain variation less than 1.6 dB.