The measurement of rotor angle makes it convenient for parameters identification of synchronous machines. The basic principle of on-line frequency response（OLFR） method is introduced in this paper. The results of identifiability analysis show that， with OLFR method， all q -axis parameters and d -axis reactance can be uniquely identified. The comparison between OLFR method and time-domain method indicates that， though two methods both have high accuracy in identification of q -axis static reactance， OLFR method is better in identification of q -axis transient parameter， and when considering the d -axis static reactance， the time-domain method has higer accuracy than OLFR method.