In this paper, an approach to estimate structural safety which may serve as a bridge between the theoretical and empirical approaches is explored. The new method combines subjective judgment and objective data to give an approximate estimate of the failure possibility. In particular, fuzzy sets are used to model linguistic of qualitative factors important to structural safety of existing buildings. By this means, vague information can be incorporated into deterministic or probabilistic models for risk estimation. The notion of possibility is consistent with the vague and subjective pronouncements common in structural assessment. Furthermore, working with possibility has computational benefits not available in probability; structural safety assessment is much simplified. Like probability, the failure possibility index for a building depends not only on the state of the building and the environment, but also on the criterion of failure. An example of a frame structure is given to illustrate the method.