The main production site of reactive oxygen species (ROS) in photosynthetic organisms is photosystem (PS) I of thylakoid membranes. Unless the suppression mechanism of ROS production functions, PSI easily suffers from oxidative damages by ROS attack. Miyake group has elucidated the production and suppression mechanisms of ROS in PSI. The reaction center chlorophyll, P700, in PSI functions in P700 photo-oxidation reduction cycle. The photoexcited P700, P700*, can donate electron to O2 producing superoxide radical, ROS, with oxidized to P700+. The accumulation of the P700+ decreases the probability of the presence of P700* not to produce ROS. The present review describes the molecular mechanism to oxidize P700 and to accumulate P700+ in PSI. Tight coupling between the light and the dark reactions in photosynthesis accumulates H+ in the luminal side and electron in plastoquinone pool of thylakoid membranes on exposure to the environmental stress, which lowers the electron transport activity of Cyt b6/f-complex and suppresses the electron flux to PSI with P700+ accumulated. We discuss the molecular mechanisms to accumulate e− and H+ and its relationship with ATP synthase activity from the aspect of P700 oxidation in PSI.