Deep-sea hydrothermal activities and hydrothermal products which were found during nearly 20 cruises in the past 26 years of DSDP/ODP are introduced. It is emphasized that deep-sea hydrothermal activity, as the study of sea-floor spreading, mantle material upwelling, new ocean-floor formation and volcanic activities, has been one of the core aspects in the Ocean Drilling Project. With the study of deep-sea hydrothermal activity, a break through progress has been obtained in many important theoretical problems.