In this paper the theory of the hypotheses evaluation about the instantaneous state of the safety-related critical processes has been described. Firstly, the critical processes (CP) are characterised. Fuzzy state images of the monitored process are defined by the composition of process variables. The hypotheses about the possible states of the safety-related critical process are set and the finite sets of the system states in discrete state space are defined. Secondly, the hypotheses are evaluated using fuzzy logic from their credibility point of view. The set of the weighted statements about the instantaneous state of the monitored process is a result.