Flexible InGaN-based green light emitting diodes (LEDs) were fabricated by transferring epilayer to a flexible polyimide substrate with laser lift-off (LLO) and double-transfer technologies. We present a method of increasing light output power in flexible LEDs without modifying their epitaxial layers. These improvements are achieved by reducing the quantum-confined Stark effect by reducing piezoelectric polarization that results from compressive stress in the GaN epilayer. The compressive stress is relaxed due to the external stress induced by increasing bending displacement of flexible substrate. The light output power of the flexible LED at an injection current of 150 mA is increased by approximately 42.2%, as the external bending went to the case of effective length of 15 mm. The experimental results demonstrated that applying external tensile stress effectively compensates for the compressive strain and changes the piezoelectric field in the InGaN/GaN MQWs region, thereby increases the probability of radiative recombination.