A method based on a unidirectional monopole ultrawideband antenna was proposed to evaluate the fluid saturation around borehole. The spectrum span of the excitation signal covered 750 MHz to 2.5 GHz and the slot antenna was optimized to transmit lateral wave more efficiently. The pulsed evaluation method offers more superiority when compared with the electromagnetic propagation tool and dielectric tools based on discrete frequencies. Measurement of complex permittivity in a wideband spectrum provides more accurate fluid saturation evaluation around the borehole. This method has been proven to operate effectively in a borehole filled with oil-based mud and to launch enough wave energy into the formation directionally. In addition, the proposed scheme is verified by using finite-difference time-domain method, and the simulation results show that this design adapts to evaluation of the fluid saturation and the detection of formation boundary. This scheme may be used as an alternative tool for logging while drilling the imaging application.