A new type of extended small signal stability region (E-SSSR) is presented in this paper so that it can consider the low frequency oscillation phenomena captured in power system operations. The definition of the extended small signal stability region is first given and its boundary structure is discussed. Then an optimal-based approach is given to trace E-SSSR's boundaries. Finally, a 3-bus and WSCC 3-generator-9-bus system are employed to do dome case studies. E-SSSR's characteristics, boundary composition and influence facts are also analyzed based on the case studies. This paper is helpful to find effective methods to avoid various small signal instabilities and low frequency oscillations in power systems.