Abstract: Pourbaix diagrams of Fe-S-H₂O hydrothermal system near 13°N on the East Pacific Rise were drawn by use of thermodynamic calculations, which illustrated changes of stable fields of predominant mineral (pyrite) in sulfide during changes from high temperatures to low temperatures of hydrothermal fluids which formed the sulfide. Mineral assemblages and chemical compositions were taken into account in computational processes. On the basis of combination with existing research on dynamic experiments and sulfur isotope fractionation, potential chemical reaction pathways which formed the predominant mineral (i.e., pyrite) during marine hydrothermal activities which precipitated sulfides were brought out. Moreover, it is suggested that the formation mechanism of pyrite in the hydrothermal system near 13°N on the East Pacific Rise has changed when hydrothermal fluids which form the sulfide change from high temperatures (>200℃) to low temperatures (25~200℃).