A pilot study of tree rings in a modern mangrove tree (Rhizophora apiculata) from Leizhou Peninsula, northern South China Sea shows that (1) the tree-rings are annual; (2) the ring widths decrease; and (3) their alpha-cellulose δ13C values increase from 1982 to 1999 AD, consistent with the trends of annual sea level, salinity and sea surface temperatures in the same period. We propose that such changes were caused by increasingly longer duration of waterlogging in response to sea-level rise. If this is the case, alpha-cellulose δ13C in mangrove tree rings can be used as a potential indicator of past sea level fluctuations.