The mechanisms for the trimerization of acetylene in the presence of (PH2CH2CH2PH2)IrCl as an active catalyst were investigated by density functional (B3LYP and M06) theory calculations. The reactant bis(acetylene)Ir complex has a distorted trigonal bipyramid structure, and thus, there are three types of equilibrium structures connected by pseudorotation. The first oxidative coupling takes place between the acetylene molecule coordinating at the axial position and that in the equatorial plane in these reactant complexes to afford square pyramidal iridacyclopentadiene complexes through one of the two favorable transition states. Then, the third acetylene molecule attacks the iridacyclopentadiene intermediate, accompanied by Cl migration, to afford the iridacyclopentadieneacetylene complex. The formation of a benzene complex from this acetylene complex involves reaction pathways based on intramolecular [4+2] cycloaddition and the Schore mechanism through iridacycloheptatriene, as well as the bicycle mechanism. The most favorable pathway for the formation of the benzene complex is through intramolecular [4+2] cycloaddition to form the η4-benzeneIr complex, and finally, to the η2- benzeneIr complex. This reaction mechanism is compared with that for reactions catalyzed by TpIr and CpIr systems.