A self-consistent Boltzmann-Poisson-Schrödinger Solver is used to study the transconductance degradation in high electron mobility transistor (HEMT), which has extensively been reported by both experimental [1]-[8] and computational [9]-[ 13] researchers. As the gate voltage of a HEMT device is increased, its transconductance increases until it reaches a peak value, beyond which, the transconductance is degraded rather sharply with further increase in applied gate bias. We previously reported a two-subband self-consistent Boltzmann-Poisson- Schrödinger Solver for HEMT. [14] We further incorporated an additional self-consistency by calculating field-dependent, energy-dependent intersubband and intrasubband scattering rates due to ionized impurities and polar optical phonons.[15] In this work, we have used our Boltzmann-Poisson-Schrödinger Solver and studied the effects of the intersubband and intrasubband scatterings of electrons, on the transconductance of a single quantum well HEMT device. The results of our simulations exhibit the same pattern reported by others [1]-[13]. We concluded that the degradation of transconductance of the device with applied gate bias is attributed to the increased intersubband and intrasubband scattering of electrons, and hence to the reduction of electrons velocity in the channel.