Although AlGaN/GaN high electron mobility transistors (HEMTs) have been attracted great attention due to its wide bandgap energy of 3.4 eV, high breakdown electric field of 3 MV/cm, superior thermal properties, and high electron mobility, few or no reports on the effect of insulator dielectric passivation layer on the breakdown voltage (Vbr) of AlGaN/GaN HEMTs have been published. In this paper, we discuss the effect of Al2O3 as an insulator dielectric passivation layer on Vbr of AlGaN/GaN HEMT. The breakdown voltage Vbr of the HEMT with a field plate (FP-HEMT) is higher than that of HEMT without a field plate (N-HEMT). The FP-HEMT with Al2O3 passivation layer exhibits the higher breakdown voltage of 860 V. It is concluded that the AlGaN/GaN HEMT with field plate and passivation layer showed improved performance and reliability.