Industrial dyeing processes produce a large amount of wastewater that contains many organic compounds such as different type of dyes and pigments, dispersing agents, surfactants which are difficult to treat. Considering that little studies have been done on biodegradation of oil-soluble azo dyes, the lake of this issue is completely felt. In this study, biotreatment of Toluidine Red (TR), an oil-soluble azo dye, was optimized under different environmental conditions. Halomonas strain Gb was capable of decolorizing TR at a pH range of 6.5-9.5 and temperature range of 25-40 °C. The optimum condition was 25 mg/L dye, pH=6.5, temperature 35 °C and 5% (w/v) NaCl. UV-Vis spectrophotometric method, high-performance liquid chromatography (HPLC) and gas chromatography-mass spectrometer (GC-MS) analyses confirmed that biodegradation of TR was accrued successfully. According to the results, Halomonas strain Gb can be recommended as practical bacteria for the treatment of industrial wastewaters containing azo dyes with different water solubility.