Sixty-eight ostracode species were found in 11 surface sediment samples collected around Kasado Bay, in the northern part of Suo-nada Bay, Yamaguchi Prefecture, Southwest Japan. Three ostracode biofacies (IB, MB and OB) were identified on the basis of Q-mode cluster analysis. Biofacies IB is distributed in the northern coastal area and contains sand and mud dwellers. Biofacies MB is distributed in the middle part of Kasado Bay, and is characterized by the dominance of Bicornucythere and low diversity indices. Biofacies OB is found in the outer part of Kasado Bay and contains relatively deep-water species. TOC and TN contents were highest in some inlets of Kasado Bay, at about 2% and 0.3%, respectively. The relationship between substrate factors (mud content, TOC and TN) and several ostracode indices (diversity, equitability, abundance, and percentages of selected species) were clarified based on calculation of their correlation coefficients. The results show ostracode diversity and abundances are negatively correlated with the substrate factors.