Upon considering the vulnerability of a urban system in a holystic way and taking into account some natural, technological and social factors, a model based upon a system of fuzzy logic, allowing to «estimate the vulnerability of any system under natural phenomena potentially catastrophic is proposed. The model incorporates quantitative and qualitative variables in a dynamic system, in which variations in one of them have a positive or negative impact over the rest. An urban system model and an indicator model to determine the vulnerability due to natural phenomena were designed.