Biological processes used for nitrification and denitrification of saline wastewater results in low treatment efficiencies because of plasmolysis or loss of activity of organisms in the presence of salt. However, adverse effects of salt on performance of nitrifcation/denitrification processes has not been systematically investigated and mathematically formulated. Biological nitrification and denitrification of synthetic wastewater containing different concentrations of salt were investigated in this study. Effect of salt concentration on the system performance was investigated. Salt concentrations above 2% resulted in significant reductions in performances of both nitrification and denitrification. Denitrification was more sensitive to salt compared to nitrification. A mathematical model was developed and the salt inhibition constants were determined using the experimental data.