The phytotechnology concept that is applied for a bio-desalination reactor become a new desalination technology. The desalination technology can be called as bio-desalination technology to remove ions of Na+ and Cl- in brackish or saline water using mangrove plant. Before the mangrove plants were used in bio-desalination technology, the preliminary phytotoxicity test was conducted. The purpose was to determine the salinity concentration at which the mangrove species of Rhizophora mucronata can survive. The preliminary phytotoxicty test was carried out using a plastic reactor that was designed as a reed bed system. The reactors filled with gravel, sand, and artificial saline water. The variation of the NaCl concentrations were 0 mg/L as control, 10,000; 20,000; 30,000; 40,000, and 50,000 mg/L. The physical observation of the survival condition of Rhizophora mucronata was carried out during the preliminary test for 7 days. The analysis of Scanning Electron Microscopy (SEM) on Rhizophora mucronata was conducted at the end of exposure. The results showed that Rhizophora mucronata could not survive at the concentrations of 40,000 and 50,000 mg/L. Rhizophora mucronata changed the color of the leaves to brown and the stems become softer. Multiple cell damage and the decreasing trend of sodium and chloride amounts occured on roots and stems at the salinity concentration of 50,000 mg/L. In conclusion, the high of salinity concentration (> 30,000 mg/L) can be toxic to Rhizophora mucronata.