Four species of diatoms from the mangroves in Fujian Province and Shenzhen City of China are described. They are Cymbella cucumis A. Schmidt, Navicula elegantoides Hustedt, N. platyventris Meister, and N. tenera Hustedt. They represent new records for China. Detailed description of the taxonomic characters of the four species and of their ecological behavior is given. Cymbella cucumis was defined as a freshwater and brackish water species for it occurred, though occasionally, where water salinity was more than 15.