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Terahertz (THz) radiation has been observed from BiFeO3 (BFO) thin films via ultrafast modulation of spontaneous polarization upon carrier excitation with illumination of femtosecond laser pulses. The radiated THz pulses from BFO thin films were clarified to directly reflect the spontaneous polarization state, giving rise to a memory effect in a unique style and enabling THz radiation even at zero-bias electric field. The emission mechanism is explained by the direct modulation of spontaneous polarization. We also find strong photoassist effect on polarization switching of BFO films.