The development of extended linear observer estimating the transient state of power system has been made previously , using one-machine infinite bus system. In this paper, the application of this estimation technique to the multi-machine system is investigated. To get the good estimation performance defining the fast convergency of estimated value to the true value, the choice of specific combination having the least number of measurments from among a large number of combinations is most desirable. The examination is made at first, in one-machine infinite bus system, next, in multi-machine system. The results of computer simulation conclude that, at least, one measurement from each electrical and mechanical system should be adopted.