Energy transfer dynamics in monomer and dimer of the photosystem II core complex (PSII-CC) was investigated by means of femtosecond transient absorption (TA) spectroscopy. There is no profound difference between the TA dynamics of the monomer and the dimer in the weak excitation intensity condition (<= 21 nJ). However, the fast recovery of the ground state bleach was pronounced at higher excitation intensities, and the excitation intensity dependence of the dimer was more significant than that of the monomer. This result indicates efficient exciton exciton annihilation taking place in the dimer due to energy transfer between the two monomer units. The annihilation dynamics was reproduced by a simple model based on binomial theorem, which indicated that although PSII-CC dimer has two reaction centers, only one charge-separated state remained after annihilation.