In this paper the state of plant is considered to be of three kinds: normal, potential failure and function failure. The time interval between brand new to potential failure and that between potential .failure and function failure are function failure are dealt with as random varibales. This paper also establishes models of functional check of plant with hidden .function failure, in which the nature and sensitivity of the output from the models are given by way of numerical examples. The time interval between brand new to potential failure and that between potential. Failure and function failure are function failure are dealt with as random varibales . This paper also established models of functional check of plant with hidden. Function failure, in which the nature and sensitivity of the output from the models are given by way of numerical examples.