The effects of strong-light photoinhibition under aerobic condition on the function and structure of Photosystem (PS) II were studied by measuring flash-induced changes of chlorophyll fluorescence yield, thermoluminescence glow curves, Fourier transform infrared (FTIR) spectra and the amounts of the PS II proteins, including the D1 and D2 proteins, in PS II membranes. Aerobic photoinhibition resulted in decrease in light-induced FTIR difference spectra of PS II. The intensity of the positive band at 1479 cm −1 due to a semiquinone anion in the Q A pocket decreased with the same kinetics as the inhibition of O 2-evolving activity. In an early stage of photoinhibition, the very fast decaying component of flash-induced fluorescence relaxation due to rapid electron transfer from Q A − to Q B was eliminated, and the intensity of thermoluminescence band due to S 2Q B − charge recombination decreased concomitant with appearance of another band due to S 2Q A − charge recombination. These results suggested that the impairment of Q B function preceded the loss of Q A function and O 2 evolution. The aerobic photoinhibition also induced a decrease in the F m level of fluorescence, but its course was h much faster than the loss of O 2 evolution. Degradation of the D1 and D2 proteins followed the loss of O 2 evolution but it did not induce clear changes in whole structure of PS II as detected by FTIR even after prolonged photoinhibition. Based on these results, the functional and structural changes of PS II by the aerobic photoinhibition were discussed, in relation to the primary event of photoinhibition.