In this pape1r, chaos phenomena and its influence on the Small Signal Stability Region (SSSR) of electrical power system are studied. A typical period-3 orbit in a simple power system is presented for the first time. Theorem Sarkovskii and Theorem Li-Yorke are used to identify the existence of chaos in power systems. This method based on sufficient theoretics can avoid the confusion caused by pseudo-chaos and incomplete period doubling bifurcation phenomena. Moreover, we discuss the influence of chaos on the small signal stability region (SSSR) and confirm that in the studies of SSSR and its boundary it is not needed from the point of engineering view to consider the existence of chaos. This excludes the unnecessary complication in the study of SSSR.