Samples for living marine phytoplankton Distephanus medianoctisol (Silicoflagellata with seven-sided basal ring) were obtained from the central Arctic Ocean during Integrated Ocean Drilling Program (IODP) Arctic Coring Expedition (ACEX) 302 which took place during summer 2004. Biometric and statistical analyses were performed on three different sample types: (1) Sea water, (2) Sea-ice, and (3) Diatom mat samples. The objectives of this study are to clarify morphological characteristics of D. medianoctisol with biometrics and statistics in three respective sample types. Out of 1226 total specimens of D. medianoctisol, 491 skeletons were measured for their length of four representative parts: radial spine, basal side, lateral rod, and apical side. The differences in environmental characteristics depending on the samples types were reflected in the length of radial spines: specimens in Sea water Group represented 1.16 times longer spines than those in Diatom mat Group. Specimens belonging to Sea-ice Group represented an intermediate spine length between those of Sea water Groups and Diatom mat. As one of the possible explanations, the difference in spine length may be due to difference in space availability in sea water and diatom mat habitats, respectively. Thus, there is a future possibility to employ the spine length of D. medianoctisol for assessing environmental conditions in which they grew.