This paper discusses the assessment of Web services trust. This assessment is undermined by the uncertainty that raises due to end-users' ratings that can be questioned and variations in Web services performance at run-time. To tackle the first uncertainty a fuzzy-based credibility model is suggested so that the gap between end-users (known as strict) and the current majority is reduced. To deal with the second uncertainty we propose a probabilistic trust approach. A series of experiments are carried out to validate the probabilistic approach built upon probabilistic databases and a fuzzy-based credibility model. The results show that the probabilistic approach improves significantly trust quality. Future work consists of incorporating several credibility models into one probabilistic trust model.