Abstract: Model uncertainty of design variables is investigatedbased on traditional stress-strength interference model in this paper. Modeluncertainty source was first studied and a random variable H was introducedto describe the uncertainty. The distribution of parameter H and itscalculation method H corresponding to different sample sizes were given.Finally, an interval stress-strength interference model with modeluncertainty was developed with.the algorithm of reliability index \beta .The results show that the proposed model in the present paper is suitablefor describing the model uncertainty.