The reaction mechanism of Br2 + I2 = 2IBr was investigated at the (U)MP2/3-21G*level, and one four-centred transition state as well as two three-atom molecules (IBr2, I2Br) were thus obtained. The same result was achieved by comparing the activation energy of three reaction paths, i.e., the minimum activation energy of the bimolecular elementary reaction Br2 + I2 = 2IBr is less than the dissociation energy of Br2 or I2. It is thus theoretically proved that the title reaction occurs more easily in the bimolecular form without light initiation. However, once the reaction system was initiated by light through the reaction path of Br atom reacting with I2 or I atom reacting with Br2, the title reaction was completed quickly, which was verified with absorbance measurement at 516 nm in the present study.