Abstract: In order to overcome the problems in the state estimation of unobservable transitions in event graph modeling for discrete event systems,the concepts of pre/post observable paths are proposed to analyze the problem in detail,and a design method is presented for the state estimator to estimate the unobservable transitions according to the row trans- formation of the incident matrix.The observation cost is analyzed and then a method is provided to search the optimal estimator,which is used to estimate the state-range of unobservable transitions.Finally an example is described to prove the effectiveness of the presented method in state estimation of unobservable transitions.