Abstract: Most of mine machines are repairable. This means that the faulted machines can be repaired and can be used continuously. Obviously, the normal or unnormal operating conditions of such machines can't be judged by a traditiogl methed with citerion "faulted machines are considered as unnormal; unfanlted machines are considered as normal". This paper presents a method for distinguishing the operating conditions of the machines by taking running time and repair time as random variables.