The automatic conformer search method for reactants is available in known software, including Open Babel , Molclus , and so on. However, to our best knowledge, the known conformer search methods are hard to be directly applied to transition states as they are either generally worked to identify the minimum points rather than the saddle points along the potential energy surface or need to set rotatable bonds manually . Therefore, it is desirable to develop an efficient and automatic conformer search method for transition states to find the most stable transition state with the lowest energy among its conformers for each rate-determining reaction. To this end, an efficient and automatic reactive site-based conformer search method for transition states is developed in this work. Its working principle is to freeze the reactive sites of transition states first, and then rotate the side chains that link to the reactive sites to find potential conformational isomers for transition states.