The quality and time execution of any production task involving human operator depend on the state of the operator and its behaviour. Recently, several studies are interested in the modelling of human behaviour in industrial system to design human centred manufacturing system. In this context, this paper focus of the fatigue of operator in production system and assume that the duration of any task will depend on his fatigue level. It proposes a model of fatigue and task duration that include this level. We developed a simulator to show the effect of the fatigue on the time duration of the task execution. We also propose a strategy to manage the level of fatigue based on the definition of the number and duration of operator rest.