In this study, an OJIP-test of two marine diatom species, Thalassiosira pseudonana (single-celled species) and Skeletonema marinoi-dohrnii complex (chain-forming species), were exposed to EC10 and EC50 levels of diuron and tributyltin (TBT) for 72 h. Increased tendency of relative fluorescence intensity (Ft/Fo value) at 300 μs and J step (2 ms) and a significantly (P < 0.05) increased Mo value (initial slope of fluorescence curve) were observed during 72 of diuron exposure in both species, whereas TBT did not increase those. These results strongly suggest that diuron efficiently blocked photosystem II-catalyzed photosynthetic electron transport at the secondary electron acceptor Q_B, which is original biological activity of diuron in diatoms. However, some parameters (e.g., ϕ_Po and ϕ_Eo, Fv/Fo and PI_ABS,) were significantly (P < 0.05) decreased by TBT treatment. Although the mechanism responsible for the decrease is not clear, relatively severe reductions in levels of Fv/Fo (an indicator of water-splitting activity) by TBT treatment than diuron treatment suggest that TBT inhibits photosynthetic function via inhibition of photosynthetic oxygen evolving systems, a different mechanism than that of diuron. Moreover, present study suggested that PI_ABS derived from OJIP-test is a high sensitive biological marker for detecting the toxic effect of pollutants which inhibit photosynthetic function.