A new concept, “Form of element in natural grain size sediments (FNG)”, was used studying the existence forms of phosphorous and silicon in five southern Bohai sea core sediments. FNG can yield the information which part of these biogenic matter contained in marine particulates can really participate in the cycling and what functions they have in the cycling, which ae two key issues dealt with in research on the cycling of marine biogenic elements. FNG provides a sound basis for research of transferable form biogenic elements contained in particulate matter. In the studied southern Bohai Sea core sediments, P exists mainly in the form of organic matter, Si exists mainly in the form of carbonate-boned. Transferable phosphorus accounts for only 17.4% (about 1.7 μmol/g) of total phosphorus and transferable silicon accounts for 0.116% (about 11.9 μmol/g) total silicon in surface sediments, which show that only a small part of P and Si in sediments can participate in the biogeochemical processes. The transferable P and Si in natural grain size marine sediments are in leachable forms, different from previously known leachable P and Si obtained from total fine grain sediments. The transferable forms of P and Si have clearly different characteristics because of rediagenesis in the total core sediments. In the deep layer of the core sediments, the stable forms can activate and become transferable forms. Therefore, the transferable P and Si in the deep layer are more than those in the shallow layers at some sites such as M9-5, M6-5, M9-12.