Prognostic of system lifetime is a basic requirement for condition-based maintenance in many application domains where safety, reliability and availability are considered of first importance. This paper presents a probabilistic method for prognostic applied to the 2008 PHM conference Challenge data. A stochastic process (wiener process) combined with a data analysis method (Principal Component Analysis) is proposed to model the deterioration of the components and to estimate the RUL on a case study. The interest of our probabilistic approach is pointed out and a comparison with existing results on the same data is made.