The purpose of this research is to develop a multi-site damage probabilistic life prediction model that could be used to assess the integrity of engineering structures susceptible to fatigue in presence of neighboring cracks. Both experiments and simulation were used to produce the data required for the model development. The experiments were performed to investigate the interaction of two adjacent semi-elliptical cracks under cyclic loading. A series of tests at different loads and for different crack aspect ratios were conducted under uni-axial constant amplitude fatigue loads on API-5L grade B steel samples. Crack growth rate of two initial semi-elliptical cracks was investigated both on the sample surface and in the depth direction. Moreover, Crack growth and interaction was investigated using a simulation technique that incorporates the stress intensity factor of a single crack with an existing cracks interaction correction factor models from the literature. Finally, a Bayesian inference modeling technique is adopted to estimate the life prediction model parameters, assess any model bias and uncertainty and validate it.