The characteristics of terahertz (THz) radiation of small-aperture spiral photoconductive antennas are studied. By using the THz time-domain spectroscopy to measure the THz radiation spectra of a spiral photoconductive antenna, its time-domain emission spectra are obtained. Then, the corresponding frequency domain spectra are obtained via fast Fourier transform. At the same time, the THz radiation characteristics of two spiral photoconductive antennas with different apertures are compared with each other. The experimental result shows that the intensity of a THz signal can be enhanced with the increase of bias voltage. For same bias voltage and pump power, the photoconductive antenna with a smaller aperture has a higher radiation power.