Uncertainties present everywhere in the physical and mathematical models. In most cases, it is difficult to describe the performance of systems using a precisely single number. But it is easy to describe the condition by using a range or an interval to represent it. In the reliability computation, when insufficient accurate data are available or the information is incomplete, a range representation of probability of failure may be more confident than the representation by a single number. In this paper, uncertainties of the parameters of stochastic model of reliability are described by intervals. A procedure is presented for the interval estimation of failure probability of mechanical structures when the distribution parameters are uncertain. It is shown by a numerical example that the probability of failure may be very sensitive to the possible errors of stochastic model. So enough attention must be paid to the reliability of stochastic modeling.