Abstract: Seamounts' formation and their substrate rock types are influenced by the distributions of fractures and magma'sJP activities. In this paper, we analyze characteristics of fractures and Cretaceous magma's activities and their influence on the formation of the seamounts in MPM and WPSP by generalizing distributions of the seamounts and geological structures, large-scale magma eruption events during Cretaceous,formation ages of the seamounts and plateaus and also composition and distribution of the rock samples in GEOROC database. The study made a conclusion that in MPM and WPSP, ocean crust was reworked by the fracture zones before the birth of the seamounts; and under the influence of the magma's activities in Cretaceous, the seamounts had two fastigiums of growth in 87~89 and 120~123 Ma or so, and their li thology characteristics are similar to those of the plateaus which were also formed in these periods.