We demonstrate a simple and compact scheme to generate ultrawideband (UWB) monocycle pulses utilizing gain saturation of a dark return-to-zero (RZ) signal in a semiconductor optical amplifier (SOA). When optical pulses with a dark RZ format propagate through the SOA, the output power at the rising edge will be overamplified compared with that at other durations due to the gain unsaturation. As a result, the output pulses are monocyclelike. The UWB frequency spectra at different injected currents, different input pulse widths, and different input wavelengths are analyzed. Our experiments show that the monocycle generation has good tolerance to the SOA bias current and the input signal wavelength.