The terahertz (THz) emission properties of two kinds of small-aperture photoconductive antennas are studied under the condition of different spot area of pump laser. The characteristic of THz radiation generated by them is compared. Meanwhile,it also provides a reference to investigate and fabricate the high efficient of THz radiation source. The emission spectra of the photoconductive antenna are obtained by THz time domain spectroscopy (TDS) technology. The corresponding frequency domain spectra are obtained by fast Fourier transform (FFT). The results show that when the bias voltage is enhanced or the spot area of pump laser becomes smaller,the peak value of THz radiation is increased and saturated with increasing of pump power. Under the same bias voltage,the spot area of pump laser and the pump power,the properties of THz radiation generated by two kinds of photoconductive antennas are compared. The results show that,the full width of half maximum (FWHM) of THz frequency domain spectra which is generated by the small-aperture bow-tie photoconductor antenna is broader and the intensity of THz radiation is larger.