The mineralogical studies on recent marine sediments collected in the western Pacific Ocean were carried out by means of microscopic, thermal, chemical, and X-ray methods. The 38 specimens which were studied can be grouped into muds, blue muds, red clays and globigerina oozes. Kinds of clay minerals were identified by X-ray, and their amounts were estimated quantitatively. Fragments of Foraminifera, diatoms, sponges, quartz, and black opaque materials consisting of magnetite or a black Mn material are found in the sandy part. Illite is the principal clay mineral in all the specimens except Globigerina oozes. Kaolinite and montmorillonite tend to be dominant in muds and blue muds (shallow-sea sediments). A chloritic mineral was also usually present in all the specimens except Globigerina oozes, and is relatively abundant in red clays (deep-sea sediments). The chloritic mineral under consideration commonly decomposes at a temperature below 600 degrees C., and as low as 450 degrees C. in some specimens.