The electron temperature of a two-dimensional electron gas heated by an electric field in DA-pHEMT heterostructures was determined by the Shubnikov – de Haas (SdH) oscillation method separately for each size-quantization subband. An analysis of the Fourier spectra of SdH oscillations showed that throughout the entire range of electron temperatures obtained, harmonics associated with different size-quantization subbands and transitions between the Landau levels of these subbands dominate in the spectra. To separate the SdH oscillations into individual harmonics, we used the approximation of the magnetic-field dependences of the oscillations by theoretical expressions using the ratio between the peak heights in the Fourier spectra. It is shown that this method allows one to determine the values of the electron temperature in case when the electron concentrations in the first and second subbands are close in magnitude. The obtained values of the electron temperature correspond to the literature data.