Although massive fish death was mostly observed in tidal rivers and mainly caused by hypoxic water mass, there was little information on DO environments in large tidal rivers. To investigate DO environments in tidal rivers with and without massive fish death, we conducted field measurements on hydrodynamics and water quality environments in the Sumidagawa and Arakawa Rivers. In summer, larger intrusion of sea water was found in Sumidagawa River than Arakawa River mainly due to the difference of river discharges, and then DO concentration in the Sumidagawa River was relatively lower than in the Arakawa River. Also found that the intrusion of hypoxic seawater mass and inflows of CSO due to hydrologic events have an important role on reduction of DO concentration in the Sumidagawa River.