Interference-fit riveting is a critical fastening technique in the field of aerospace assembly. The fatigue and sealing performance of the rivet joint are determined by the interference-fit level of the rivet joint. As a result, it is of great importance to measure the interference-fit level accurately and effectively. Conventional interference-fit level measurement methods can be divided into direct measurement (destructive test on test-piece) and indirect measurement (off-line dimensional measurement of upset rivet head). Both methods cannot be utilized in automatic riveting. In this paper, an on-line non-destructive measurement method is developed to measure the interference-fit level. By taking full advantage of servo-driving riveting integrated with force measurement, the force-deformation data of the deformed rivet can be obtained in real time. The recognized feature points from the force-deformation data can reflect the height of the upset rivet head. An algorithm to calculate the diameter of the upset rivet head based on the recorded maximum riveting force is developed. So as to obtain the volume of the upset rivet head. Thereafter, the volume of the rivet shank in hole can be obtained, and the interference-fit level of the rivet joint can be calculated online due to the law of volume constancy. More importantly, to realize real time calculation, an on-line data processing software in C++ program language is developed and integrated into the HMI software of the control system. At last, both finite element simulation and riveting experiment are conducted to validate the feasibility and effectiveness of the proposed methodology.