We propose and demonstrate a novel approach to optically generate ultrawideband (UWB) monocycle pulses by exploiting the parametric attenuation effect of sum-frequency generation (SFG) in a periodically poled lithium niobate (PPLN) waveguide. The SFG process changes the continuous-wave pump into dark optical pulse pump with undershoot, resulting in the generation of UWB monocycle through the combination of input signal and output pump with proper relative time advance/delay. Pairs of polarity-inverted UWB monocycle pulses meeting the UWB definition of U. S. Federal Communications Commission (FCC, part 15) are successfully obtained in the experiment.