Abstract: Diatoms and grain size of surface sediment samples from the Western Philippine Basin were analyzed, and 36 species belonging to 21 genera were identified, showing rich diversity and wide distribution character. The dominant diatom species included Azpeitia nodulifera A. Schmidt, Aulacoseira perglabra (Oestrup) Haworth, Aulacoseira lacustris var. nivalis (Smith) Haworth, Aulacoseira distans var. nivalis (Smith) Haworth, Aulacoseira distans (Ehrenberg) Simonsen, Aulacoseira canadensis (Hustedt) Simonsen, Aulacoseira lacustris (Grunow) Krammer, Coscinodiscus africanus Janisch, Cyclotella striata (Kuta.) Grunow, Thalassiosira excentrica (Ehr.) Cleve, and Ethmodiscus rex. The key factors on diatom abundance were investigated. The southeastern part of the study area features gentle terrain, weak hydrodynamic forces, stable sedimentary environment, and the proximity to the North Equatorial Current. These conditions resulted in nutrient-rich seawater and sediments dominated by fine-grained components such as pelagic clay and silt, leading to high diatom abundance. In contrast, the northwestern part is obstructed by a ridge, where strong water currents cause intense scouring, resulting in coarser sediment particles, which are unfavorable for diatom growth and settling.