GaN buffer with insertion of a 40-nm-thick high-temperature （HT） AlN interlayer is studied. The HT-AlN interlayer enhances the in-plane compressive strain of GaN film and thereby improves the electrical properties of AlGaN/GaN high electron mobility transistors （HEMTs）. Base on the precise measurement of Bragg angle, the strain states of GaN are calculated. It is found that the increased compressive stress enhances the piezoelectric polarization field in GaN, which consequently causes accumulation of more electrons at the AlGaN/GaN interface. On the other hand, the influence of AlGaN layer induced by the enhanced compressive stress on the two-dimensional electron gas （2DEG） sheet carrier density, both positive and negative, are proved to counteract each other. Meanwhile, the employment of the HT-AlN interlayer reduces the lattice mismatch between the GaN and AlGaN and smoothes the AlGaN/GaN interface, thus increases the 2DEG mobility by weakening the interface roughness scattering. The 1-μm gate-length HEMTs by using the GaN buffer layer with the HT-AlN interlayer are fabricated. The measurements show that the maximum drain current and transconductance are increased by 42% and 20% respectively compared with the conventional HEMTs without HT-AlN interlayer.