Although the reliability of flexible electronics during bending deformation is of great interest nowadays, the mechanical reliability that has mainly been investigated is of single electronic components or simple devices, such as metal interconnect, transparent conductive electrode, or thin-film devices, rather than that of the real package sample having complex structure and various materials. This study systematically investigated the mechanical reliability of flexible Si package sample consisting of Si die, polymer bump, and polymer substrate, by using individual resistance monitoring of the metal line, bump array, and total interconnect. For the bending test, the sample consisting of only Si die and polymer substrate shows abrupt electrical resistance increase below a bending radius of 3 mm, due to cracking of the Si die. For the bending fatigue test, the electrical resistance increases after 2000 cycles in 5 mm bending radius, due to fatigue failure of the metal line and bump array. Both the maximum bendability and fatigue lifetime can be significantly improved by covering with the molding layer. Finite element method simulation is conducted to analyze the mechanical stress distribution of the flexible package with and without molding layer during bending deformation. This study based on experimental results and simulation analysis can provide helpful guidelines for the design of highly reliable flexible packages.