For characterization of semiconductor lasers, quasi-Fermi-level separation is a critical parameter due to its relationship with carrier density and gain. Its determination should be made with accuracy within ±10 meV. In this letter, we report a simple technique to accurately measure the quasi-Fermi-level separation, photon energy at gain peak, and intrinsic optical loss for a given current. Through theoretical calculation and experimental results, we believe that the quasi-Fermi-level separation measured by our technique is accurate to ±0.5 meV. © 2000 American Institute of Physics.