Due to the importance and wide applications of xylenes, as well as large investments in their production processes, which has increased significantly since 2001 and their greater compatibility with the environment (lower soot production during combustion) compared to heavy aromatic compounds such as trimethyl benzene, this subject has been investigated Xylene isomers were synthesized by trans-alkylation of toluene and a sample of Isfahan petrochemical plant (T-202) product that contains 45% of total trimethyl benzene isomers has been investigated in sub/supercritical toluene using a continuous system. In order to reduce experiment time, cost of investigations and evaluation of the effects of different factors, Minitab software was used to design the experiments. After experimental design by Minitab software, trans-alkylation and disproportionation reactions were performed under sub/supercritical toluene in the presence of beta zeolite as catalyst (SiO 2 /Al 2 O 3 = 35) and optimal conditions were obtained. The results show that trans-alkylation reaction of toluene and trimethyl benzenes in supercritical toluene (at temperatures greater than 318 °C and a pressure greater than 42 bar) is done better, due to the properties of the supercritical fluids. Moreover, by increasing the amount of trimethyl benzenes to toluene up to 50%, trans-alkylation reaction is taken place better than disproportionation. In general, the following results were obtained from this thesis: The yield of Xylene production is in the following order: trans-alkylation reaction of trimethyl benzenes Toluene disproportionation of toluene at supercritical and subcritical conditions.