Higher plant photosystem Ⅱ (PS Ⅱ) generates superoxide anion radicals (O₂^·) under strong illumination, in which the O₂^· plays a pivotal role in reactive oxygen species (ROS) metabolism during higher plant photosynthesis. The photosystem of Chlamydomonas reinhardtii is highly similar to that of higher plants. The generation of O₂^· in C. reinhardtii thylakoid membranes and PS Ⅱ particles has been firstly proved by means of spin trapping-ESR technique and NBT-mediated spectral assay. Referring to the spin trapping-ESR evidences in spinach, the mechanisms of O₂^· generation both in C. reinhardtii and spinach look identical. Compared to higher plant, C. reinhardtii has several irreplaceable advantages: simpler structure; clearer genetic background and genomic sequencing has been finished; easy to introduce mutations in photosystem. As a result, the study concerning the O₂^· generation in C. reinhardtii introduces a new promising model for further investigation of ROS metabolism in higher plant photosynthesis.