Abstract: The in-site material strength of existing structures is an uncertain quantity which is objectively certain but subjectively uncertain. The current interval estimation method is a probability inference approach, which is not suitable for the estimation of realistic material strength of existing structures. Based on the characteristics of existing structures, the paper analyzes the flaw of interval estimation method and then proposes the belief inference method for the estimation of material strength of existing structures. Based on reliability theory, the belief inference method can reflect the effect of measurement uncertainty and space uncertainty to inference results. Moreover, the difference between tested and untested structural members is taken into account. Exampled analysis results demonstrate that inference results are consistent with the engineering judgment. Moreover, the method could also be extended to infer the in-site permanent loads of existing structures.