Graphitic carbon nitride (g-C3N4) and sulfur-doped g-C3N4 were prepared by pyrolysis of melamine and thiourea respectively. Their comparative performance was investigated for photo-degradation of a Rhodamine B (RhB) an organic toxic pollutant. The crystal structure, morphology, microscopic components and properties of the synthesized samples were characterized by XRD, TEM, FT-IR, photoluminescence (PL) emission spectroscopy and zeta potential. TG-DTA is a record of the process for pyrolysis of thiourea. Two simplified kinetic models, pseudo-first-order and pseudo-second-order were applied to predict the adsorption rate constants. Thermodynamic parameters, such as the change in free energy, enthalpy and entropy were also calculated to analyze the process of adsorption. Adsorption isotherms and equilibrium adsorption capacities were established by three well-known isotherm models including Langmuir, Freundlich and Dubinin-Radushkevich (D-R). Both samples were investigated for underlining the reaction mechanism during the photodegradation RhB process and then can be assigned to the overall reaction. The photosensitive hole is regarded as main oxidation species for the degradation by sulfur-doped g-C3N4, but not the exclusive way for g-C3N4. It is worth mentioning that the optimum operating condition can be obtained by orthogonal experiments.