Thermally activated longitudinal optical (LO) phonon scattering time (τtherma LO) of excited subbands in a 3.8THz quantum cascade laser is studied. The active region used herein is based on a LO phonon depopulation scheme. A single plasmon waveguide was used for optical confinement. By calculating the temperature dependence of threshold current density (Jth) and comparing with the experiment, τ therma LO limiting the characteristic temperature of Jth is estimated: τtherma LO is 246 ns at 5 K, which rapidly decreases 15 ps when temperature rises up to 100 K. © 2009 The Japan Society of Applied Physics.