Abstarct: Herein, the tri-cyclotrimerization mechanism of acetophenone with the aid of Density Functional Theory (DFT) in the absence and presence of H2SO4, H3PO4, TiCl4, and FeCl3 as catalysts has been performed. These catalysts showed the highest experimental rate constants. The mechanism includes the rearrangement of acetophenone because of its protonation and then nuclophilic attack of acetophenone to its protonated form. These catalysts showed the highest experimental rate constants. The proposed mechanism proceeds through three rate determining steps. These steps contain proton transformation and formation of protonated cycle. Therefore, three transition states (TS1, TS2, and TS3) in the absence and presence of mentioned catalysts have been determined. The catalysts with ability of proton catching increase rate of reactions and decrease activation energy barrier. Our results show that FeCl3 and TiCl4 catalyze the mechanism with lower activation energy. All calculations were executed with Gaussian 09 using B3LYP with LANL2DZ and 6-311+g(d,p) basis sets for transition mextals and other atoms, respectively.