This paper describes a dependability analysis algorithm for degradable fieldbus system.Our algorithm is valid for a general problem model:The system contains several performances (each performance consists of several particular functions)

and can be at different performance at the beginning of work.In addition

the system's performance will be changed by some faults as it is working.Finally

the structure of system is scalability.In this paper

reliability and availability are connected to evaluate the dependability of system.Another difference between our algorithm with previous algorithms is that our algorithm is based on system performances.