Abstract:Causality Diagram Theory is a kind of uncertainty reasoning model based on probability theory and can be very much effectively diagnose and analyze the faults of a system, in causality diagram model, however, the occurring probability of an event is required to be accurate value but in practical life, the occurring probability of an event has the feature of fuzziness and uncertainty. This paper introduces fuzzy set theory into the diagram, extends the ordinary events to fuzzy events, furthermore, makes quantitative and qualitative analysis of fuzzy causality diagram, eases the difficulty in getting the accurate value of occurring probability of an event so as to let the application range of the causality diagram wider.