The problem of a corner delamination in a fan-out chip scale package under thermomechanical load is investigated. The fracture mechanics parameters, including the stress intensity factors and the strain energy release rate, for a quarter-circular corner delamination between silicon die and fan-out redistribution polyimide layer are obtained by using numerical finite element approach with 3-D virtual crack closure technique (VCCT). Results of the analysis indicated that contact between crack faces occurs under temperature cycling condition. The delamination driving forces are mode-II and - III dominant. In addition, the strain energy release rate is highest near the location where the delamination crack front intersects die-to-molding compound interface. Parametric study is also conducted to investigate the effects of material properties and geometrical dimensions on the delamination driving forces. The calculated fracture mechanics parameters may be combined with the experimental data on the resistance to interface fracture for predicting reliability of the package.