Abstract: The Cretaceous is an important period in which occurred many geological events, especially the OAEs (Oceanic anoxic events) characterized by black shales, and the oxic process characterized by CORBs (Cretaceous oceanic red beds). This paper describes the causative mechanism which explains how the oceanic environment changed from anoxic to oxic in Cretaceous. Two typical events show different results caused by the same reason. On the one hand, the Cretaceous large-scale magmatic activities caused CO₂ to be concentrated in air,the inner energy of the Earth to release and the ocean-land area to change. Finally, they caused the increase in atmospheric temperature. This change presented the same trend as the oceanic water temperature, and caused the decrease of O₂ concentration in the Cretaceous ocean, and then the OAEs occurred. On the other hand, the violent volcanoes supplied lava containing Fe for the seafloor. When the seawater reacted with the lava, the element Fe became dissolved in seawater. Iron is a micronutrient essential for the synthesis of enzymes required for photosynthesis in oceanic environment, and it could spur phytoplankton growth rapidly. The growth of phytoplankton which can consume carbon dioxide is commonly found in the world's oceans, wherever they are in atmosphere or in ocean. This process could produce equal oxygen. And then, the oxic environment characterized by red sediment rich in Fe³⁺ appeared. The anoxic and oxic conditions in the Cretaceous ocean were caused by magmatic activities, but they were of different causative mechanisms. The former was based on physical and chemical process, while the latter involved more complicated bio-oceanic-geochemistry process.