The controlling factors affecting the accumulation of (137)Cs in marine sediment have not been investigated in detail, especially in coarse grained sediment. Eighty eight coarse marine sediment samples near Wuljin, Korea, were characterized by quantitative X-ray-diffraction (XRD), gamma-ray, and total organic carbon (TOC) analysis. Those factors were then compared. The grain size was in the range of -0.48 to 3.6Mdphi corresponding to sand grains. TOC content was in the range of 0.06-1.75%, and the concentration of (137)Cs was <MDA to 4.0Bq/kg-dry. The main identified minerals were general rock-forming minerals such as quartz, feldspars with minor contents of pyroxene, calcite, hornblende, a 10A phase of phyllosilicate assigned to biotite, and chlorite. Other clay minerals were not identified due to the large grain sizes of the investigated samples. Biotite (1-7wt%) was the only mineral showing a positive correlation with (137)Cs activity, which was first reported here, probably due to the weathered frayed edge site of biotite produced by a release of K. The samples with low TOC contents showed even better correlation between biotite content and (137)Cs activity. For the entire samples, however, the TOC content showed better correlation with (137)Cs activity than other single factors, indicating that biotite and organic carbon are the most important factors controlling (137)Cs fixation. The combined effect of biotite and TOC for (137)Cs fixation was also confirmed by multiple regression analysis ((137)Cs activity=1.712.TOC (wt%)+0.202.biotite (wt%)-0.097; R(2)=0.819). The regressed slopes indicated that the (137)Cs-adsorption capacity of TOC was about 8.5 times higher than that of biotite. However, the amount of (137)Cs adsorbed onto biotite was 30% more than that adsorbed onto TOC due to much greater biotite content in the sediment. The role of biotite in fixing (137)Cs becomes more important in sediment with coarser grains, containing little TOC.