Supercritical fluids have specified characteristics which make them an appropriate media for chemical reactions. To carry out chemical reactions in supercritical conditions, achieves specific advantages such as changes in reaction media (solubility behavior) with pressure, increase in solubility of reactants and products omission of mass transfer restrictions in single phase and superposition of reaction and separation units. In this thesis an appropriate mathematical model was used for the phenol oxidation in supercritical water and the reactor was simulated in order to avoid experiments in new operating conditions. For this purpose the characteristic of supercritical water as a reaction media was investigated. For model extension, supercritical oxidation of phenol with and without catalyst was studied. Considering the fact that in the experimental investigation up to now and ideal tubular steady state reactors have been used therefore the same kind of reactor was assumed in the mathematical modeling of this study. To investigate the authenticity of the mathematical model the experimental data for the oxidation phenol at supercritical condition was compare with the theoretical results of this research. Base on this comparison among the noncatalytic mechanism the network model by "Goppalan Savage" was very close to the model data. The deviation for the variable of phenol conversion percentage was about 5% . the experimental results for the phenol conversion in the catalytic supercritical oxidation using CuO/Al 2 O 3 compare with the modeling data and 2% deviation was observed, Which shows that the mathematical model for catalytic reactor with respect to the noncatalytic reactor model is closer to the real condition. Using the power low reaction rate in the mathematical model predicted much closer data compare to other rates such as "Lungmuir-Hinshelwood-Hougon-Watson". In the noncatalytic supercritical oxidation, reaction between reactants is the slow and controlling step. Mean will in the catalytic supercritical oxidation, adsorption on the active sites is obvious and for this reason "Langmuir-Hinshelwood" equations were used in the mathematical model. It is imperative to realize that only for very high active catalyst, pore diffusion resistance is the controlling step and it was proved that "CARULITE 150" catalyst has this mass transfer property. Comparison between supercritical oxidation modeling data with subcritical oxidation modeling data indicates that supercritical oxidation with respect to subcritical oxidation has higher conversion percentage, less byproducts, noncomplicated network models and much better fit with the obtain experimental data. Therefore it is concluded that the developed mathematical model in this research is viable method for predicting real experimental data and the model's prediction is well within the experimental data and accuracy.