Although Li metal batteries (LMBs) with high flexibility are attractive energy storage systems for wearable devices due to their high energy density, there exist critical challenges such as limited loading density, poor dimensional stability, and thus mechanical instability under repeated mechanical deformations including bending, twisting, and folding. In this work, we developed extremely flexible nano/microfiber composite electrodes for LMBs. The highly flexible and mechanically durable electrode was composed of oxidized PET microfiber substrates covered by single-walled CNT nanofibers, prepared by mass-producible O₂ plasma treatment and spray-coating.  CNT nanofibers provided an electrical intertangled network to integrate the active materials (i.e. either NCM or PLP) and anchored to a flexible O-PET MF substrate, which permits high mass loading of active materials, improved electrochemical performance while maintaining mechanical durability. Resultantly, the flexible LMBs based on nano/micro fibrous electrodes have excellent energy density (300.1 Wh kg^–1 electrode; nominal voltage of 3.7 V) and stable capacity retention (97.1%) without mechanical failure over 500 folding cycles.