Abstract: The borehole radar is a new method in borehole geophysical detection. In order to reduce the antenna structure size and increase the bandwidth, we designed a new resistive loading omni-directional antenna according to borehole radar requirements. Through theoretical analysis and numerical optimization, we selected the better structure, the antenna had good radiation pattern and bandwidth. The outside diameter of the antenna was 65 mm. The outer diameter was 85 mm, using a 10 mm tube encapsulated. S11 of the antenna was less than -10 dB in the frequency band 70~260 MHz, with 190 MHz bandwidth. The center frequency gain was greater than -3 dB, and the radiation electric field pulse waveform tailing was less than 1/5 of the main peak amplitude, the pattern in the H-plane was omnidirectional. The measurement results of antenna characteristics using the vector network analyzer agree well with the numerical results. The results show that the design of the antenna can meet the requirements of the small size of broad borehole radar system.