In this paper, the interfacial fracture behavior of a flip-chip package subjected to a constant concentrated line load was investigated using a unique 6-axis submicron tester coupled with a high density laser moiré interferometry. The real-scale three-point bending flip-chip specimen, capable of measuring the crack growth rate (along the interface) and the interfacial fracture toughness was developed. The results show that the crack propagation along the interface of the passivated silicon chip/underfill under a constant concentrated load can be categorized into three stages: (1) stable crack propagation stage, (2) unstable crack propagation stage, and (3) quasi-crack arrest stage. There exist two obvious transition points between the stable crack propagation stage and the unstable crack propagation stage, and between the unstable crack propagation stage and the quasi-crack arrest stage. The moiré interferometry technique was used to monitor and measure the crack length during the test. The crack growth rate along the interface of the passivated silicon chip/underfill was calculated in terms of the load line deflection vs. time curve obtained from the test. In addition, the relationship between the crack length and the load line deflection was calibrated by using finite element analysis. The near tip displacement fields of the flip-chip package was also determined by the same method. The energy release rate was computed by using these near tip displacement variables through an analytical expression derived by authors. The interfacial fracture toughness Gc was determined by the energy release rate corresponding to the crack length at the quasi-crack arrest stage measured in the test. The interfacial fracture toughness Gc and the phase angle ϕ of the flip-chip package considered at the interface where the passivated silicon chip meets the underfill are about 35 J/m2 and −65° respectively.