An approach has been developed to measure the internal electrical field (Einternal) in InGaN quantum wells of GaN LEDs, which is vital for the internal quantum efficiency of GaN LEDs. Firstly, the initial forward voltages, Vf0, of the studied GaN LEDs were measured to be 3.1579 V with pulse mode at a current input of 350 mA. A linear relation between the forward voltage change and the stressing voltage was established in this work. Voltage-stressing GaN LED with a large reverse bias, −7 V, we found that the initial compressive stress in MQWs was converted to be in tensile state. A stress-free condition (i.e., flat-band condition) occurs at the exact transition point of the compressive-to-tensile stress conversion. The stress-free condition implies the maximum Vf.max (flat-band condition) of the studied GaN LEDs, which can be defined to be 3.1597 V with the forward voltage measurement against relaxation time. The difference between Vf0 and Vf,max (flat-band condition) is approximately 2 mV. Plugging 2 mV in the above obtained relation between the forward voltage change and the stressing voltage, the flat-band voltage is determined to be a reverse bias of 1.18 V. With Renner's equation, the initial Einternal across the In0.15Ga0.85N quantum wells can be calculated to be 1.03 MV/cm. The magnitude of the present calculated Einternal agrees well with the reported Einternal, which is in the range of −0.92 to 2.04 MV/cm.