Abstract: We set up three sampling stations on the mudflat in Shenzhen estuary to investigate free-living marine nematodes. Twenty-eight species of marine nematodes were obtained during four surveys. These nematodes belong to 22 genera , 13 families and three orders. The dominant species were Terschellingia sp1. , Daptonema sp1. , Metalinhomoeus sp. and Paradontophora sp. The density of marine nematodes was the highest in spring , the second in winter , the third in summer and the lowest in autumn. Thepattern of marine nematode assemblage was similar to that of the macrofaunal community on the mudflat in Shenzhen estuary. For example , both had low species number , low species diversity index and low richness index , and the dominant species appeared with high population density. The seasonal fluctuation of the nematode abundance showed almost the same pattern as the abundance of the polychaete , Tharyx sp. It appeared that the most dominant species of nematodes had a type of symbiotic relationship with Tharyx sp. This phenomenon may relate to their ecological habit because they live and breed in mudflats with high content of organic matter. Key words 　marine nematode , species composition , mudflat , Shenzhen estuary