The palladium-catalyzed Mizoroki-Heck reaction is one of the most widely used and important reactions in organic synthesis and organometallic chemistry. Since the ligand of Pd complex used as a catalyst influences a reaction, the search for ligands is one of the important work. In this study, we theoretically investigated the reaction mechanism of Mizoroki-Heck reaction with PdCl₂(2-(2'-pyridyl) benzo azole) catalyst using the DFT method. From the calculated results, the active species was found to be Pd(Ph)(Br)(L) complex which produced by the reactive substrates. There are consisted of seven processes in this reaction mechanism, as follow: (1) ethylene coordination, (2) ethylene Insertion, (3) β-H Abstraction, (4) styrene elimination, (5) Ph-Br coordination, (6) H-Br elimination, and (7) Ph-Br oxidative addition. The rate-determining step is Ph-Br oxidative addition, and the values of the activation free energies were calculated to be 30-32 kcal/mol. From these results, it was suggested that this catalytic cycle is proceeded by Pd⁰/Pd^II, not Pd^II/Pd^IV.