Reliability assessment of the digital substation automation system（DSAS） is an indispensable prerequisite for its large-scale application. An assessing method including functional reliability and system reliability evaluation of DSAS is pro-posed. This method takes into account the reliability of software and hardware of intelligent electronic devices（IEDs）， commu-nication system reliability， and communication delays. The DSAS is presented as a graph in which the apexes stand for IEDs and switches， edges for communication links. According to thework” andfailure” states of each element， all states of DSAS can be obtained. By means of functional integrity and delay constraints， the DSAS states can be derived and classified into two kinds， namely，work” andfailure”， and allfailure” states can be merged into one state. Then the reliability of DSAS func-tions and system is modeled using the discrete-time Markov chain. In addition， the method of calculating the state transition matrix and that of calculating the time prior to mean failure are derived according to the state transition matrix and initial state.