Event tree is a logical analysis of probability for outcome events by initiating event and probable subsequent events. However, the evaluation of every single event may contain vague information or cannot be described by a crisp number of probabilities in application. So this paper aims to develop an event tree by using fuzzy set theory with a combination of triangle membership function and trapezoidal membership function to quantify the vagueness of information and calculate the outcome probability. Through the analysis of an oil pipeline leakage accident, the method provided is fully used and the expansion of accident can be tracked through different paths of event tree.