This book describes and illustrates 300 species of free-living marine nematodes from the East China Sea and includes eighteen new species. Free-living marine nematode is the most dominant and diverse meiofaunal group in marine benthic habitats. It has strong adaptability and wide distribution, and plays a very important role in the material circulation and energy flow of benthic ecosystem. Up to now, about 7,000 species of free-living marine forms (attached to 2 Class, 8 Order, 86 Family, 662 genera) have been recorded around the world. Some 500 species have so far been reported from the sea areas of China. Among them, more than 300 species were identified from the East China Sea. The book will provide basic data and information of free-living marine nematodes for ecosystem management, protection and utilization of marine biological resources in the East China Sea.