Future　electronics　would　be　conformal,　bendable　and　wearable,　which　requires　the　flexible　devices　possess　exceptional　electronic　properties　and　mechanical　stability　during　mechanical　deformation.　However,　in　flexible　electronic　systems,　the　quantitative　characterization　of　interface　stress　between　layers,　which　significantly　impacts　the　mechanical　properties　and　electronic　performance　of　flexible　devices,　is　still　not　available,　which　limits　the　optimization　of　device　performance　and　deep　understanding　of　the　relevant　mechanism　in　flexible　electronic　devices.　Here,　for　the　first　time,　a　nanoindentation　technique　is　introduced　to　precisely　characterize　the　interface　stress　of　flexible　organic　field　effect　transistors　(OFETs),　which　are　also　broadly　applicable　for　other　flexible　electronic　devices.　Moreover,　the　effect　of　interface　stress　on　the　mechanical　stability　of　flexible　OFETs　is　investigated.　Noticeably,　the　results　first　illustrated　that　interface　stress　could　be　tuned　by　controlling　the　interface　adhesion　energy　between　layers.　The　improved　interface　stress　would　directly　decrease　the　mechanical　damage　to　the　insulator/semiconductor　interface　charge　transfer　during　bending　deformation　and　eventually　improve　the　mechanical　stability　and　electrical　performance　of　flexible　devices.　This　study　clearly　demonstrates　that　the　nanoindentation　technique　is　a　promising　tool　to　precisely　characterize　the　interface　stress　of　flexible　electronic　systems　and　provides　guidelines　for　the　fabrication　of　high　performance　flexible　electronics.　©　The　Royal　Society　of　Chemistry　2020.