Femtosecond transient absorption spectroscopy was applied to the photosystem I reaction center particles containing only 12-13 chlorophylls per primary electron donor chlorophyll (P700), which were prepared from spinach. Upon preferential excitation of P700, an absorption band due to the excited state of P700 was newly observed in the 730-750 nm region. The primary charge separation dynamics at approximate to 280 K were revealed by the rise of the absorption band of the radical pair of chlorophylls. The rise was biphasic: 0.8 ps (50%) and 9 ps (50%). The 0.8 ps component is the fastest phase of the charge separation ever observed in photosystem I reaction center. On the basis of the observed dynamics and a kinetic model of excitation migration, the rate constant of the charge separation between P700 and the nearest-acceptor chlorophyll is predicted to be (0.5-0.8 ps)(-1).