High-performance mechanically flexible Si nanomembrane (NM) fully depleted silicon-on-insulator field-effect transistors are realized via neutral mechanical plane (NMP) optimization. This NMP-optimized Si NM flexible device, using both the analytical and numerical modeling, shows excellent mechanical and electrical stability even at a bending condition with a 1 mm radius. The strain at this point is less than 0.01% that is much smaller than the strain tolerance of 0.1%. This work reveals that mechanical reliability is heavily associated with the location of the NMP and the NMP optimization is essential to realize the Si NM flexible electronics.