It is important to study the quantified analysis technique about the property of electromechanical oscillation mode and the participation feature of generator, for developing the situational awareness technology for the large scale power systems. Therefore, this paper presents an improved quantitative indicator for the relative localness of electromechanical oscillation mode, denoted by Lidx. Then the impacts of different boundary conditions such as generator type, power flow of tie line, and spinning reserve capacity on the modal localness of and the participation feature of generator electromechanical oscillation mode have been studied. The case studies have been carried out on a Kundur 4-machine and 2-area system and its variants, utilized by PSS/E V33, and the results verify the effectiveness of Lidx which could be helpful to track the evolution of inter-area mode or local mode. Moreover, it indicates that the participation feature of generator would be changed and consequently modal property would be transformed from local mode to inter-area mode or vice versa if the boundary condition is significantly varied for instance the fluctuations from load consuming or power generating and the integration of doubly-fed induction generator (DFIG). Furthermore, it is possible to predict the generator with maximum participation for inter-area mode when the DFIG is integrated into a regional power grid.