An optimal placement method of rotor angle transducers for power system stability is presented in this paper. For angle stability, information content of generator rotor angle responses to disturbances is an appropriate performance criterion of placement sites. To maximize information content, the method consists of three steps: clustering, grouping and selecting. After distinct dynamic scenarios are extracted by a hierarchical agglomerative clustering technique, a coherency identification step is implemented to form candidates and then placement sites are determined using Gram determinant as the information content measure. The proposed method is applied to a transient stability emergency control scheme in a practical power system. Simulation results demonstrate its rationality.