The subauroral polarization streams (SAPS), associated with strong electric fields in the subauroral region, play an important role in energization and transporting electrons and ions in the inner magnetosphere. A new mechanism for SAPS generation is considered. The mechanism is based on a self-consistent solution for plasma convection in nightside magnetosphere. In accordance with most researchers, we suggest that SAPS appears between the ion and electron Alfven layers in the nightside magnetosphere. However, our model for the SAPS generation does not require the ion Alfven layer to be located closer to the Earth than the electron Alfven layer. Rather the opposite, we assume that due to lower temperature of plasma-sheet electrons, the electron Alfven layer at the nightside is located predominantly if not entirely inside the ion Alfven layer. This mechanism explains the generation of the SAPS in a wide longitudinal sector from the evening to post-midnight, and the magnitudes of potential drop across the SAPS region and the electric field in the stream are well consistent with experimental data. We also obtained simple formulas deriving the parameters of the polarization stream.