Abstract: The occurrence of Cretaceous Oceanic Red Beds (CORBs) suggest an oxic ocean in Late Cretaceous. Based on the data from the modern ocean iron release experiment, we discussed in this paper the function of iron in the oceanic oxic proces and the mechanism of oceanic environment change from anoxic to oxic in Cretaceous. Submarine volcanism provides enomous basic and ultrabasic lava, which contains a certain amount of Fe, onto the seafloor. When the lava react with seawater, the element Fe will be released to and dissolve in seawater. Iron is a micronutrient essentially required by the synthesis of enzymes during photosynthesis in the oceanic environment, and will help phytoplankton boom rapidly. The booming of phytoplankton needs carbon dioxide and will take it from the nature, wherever it is in atmosphere or in ocean. This process will produce a great amount of oxygen, and cause the formation of an oxic environment characterized by red sediment rich in Fe³⁺. The forming mechanism of the CORBs is supposed to be a complicated bio-oceanic-geochemical process related to submarine magmatic activities.