Dietary inorganic sulfur is the minor component in our diet, but some studies suggested that inorganic sulfur is maybe effective to treat cancer related illness. Therefore, this study aims to examine the effects of inorganic sulfur on cell proliferation and gene expression in MDA-MB-231 human breast cancer cells. MDA-MB-231 cells were cultured the absence or presence of various concentrations (12.5, 25, or 50 µmol/L) of inorganic sulfur. Inorganic sulfur significantly decreased proliferation after 72 h of incubation ( P < 0.05). The protein expression of ErbB₂ and its active form, pErbB₂, were significantly reduced at inorganic sulfur concentrations of 50 µmol/L and greater than 25 µmol/L, respectively ( P < 0.05). The mRNA expression of ErbB₂ was significantly reduced at an inorganic sulfur concentration of 50 µmol/L ( P < 0.05). The protein expression of ErbB₃ and its active form, pErbB₃, and the mRNA expression of ErbB₃ were significantly reduced at inorganic sulfur concentrations greater than 25 µmol/L ( P < 0.05). The protein and mRNA expression of Akt were significantly reduced at an inorganic sulfur concentration of 50 µmol/L ( P < 0.05), but pAkt was not affected by inorganic sulfur treatment. The protein and mRNA expression of Bax were significantly increased with the addition of inorganic sulfur concentration of 50 µmol/L ( P < 0.05). In conclusion, cell proliferation was suppressed by inorganic sulfur treatment through the ErbB-Akt pathway in MDA-MB-231 cells.