We determined the energy states of a two-dimensional electron gas (2DEG) in high electron mobility transistors (HEMTs) with a pseudomorphically strained InAs channel (InAs PHEMTs) rigorously, taking into account the nonparabolicity of the conduction band for InAs. To consider this nonparabolicity in the Schrödinger equation, we have used the energy-dependent effective mass based on Kane's k · p perturbation theory. The Schrödinger equation was solved according to a standard perturbation theory by regarding the terms including the nonparabolicity parameter as a perturbed Hamiltonian. Numerical simulation was carried out by solving the Schrödinger and Poisson equations self-consistently. Since the effective mass of electrons increases with their energy, the Fermi energy EF and the first and second subband energies lie within the InAs well and hence most of the electrons are strongly confined in the InAs well. When the same calculation was done for conventional In0.53Ga0.47As HEMTs for comparison, there was no noticeable difference between the results for the parabolic and nonparabolic conduction-band cases. In addition, it was found from the ns dependence of EF that the threshold voltage VTH for InAs PHEMTs is about 0.21 V lower than that for conventional In0.53Ga0.47As HEMTs. This VTH shift corresponds to the effective conduction-band offset energy between the In0.52Al0.48As and InAs layers. The theoretical result for VTH agrees fairly well the experimental ones reported so far.