Terahertz optical and dielectric properties of AlN ceramics fabricated by hot pressed sintering are investigated by THz time-domain spectroscopy in the frequency range of 0.2-3.5 THz. The measured properties of the pure AlN ceramic are compared with those of Y2O3-added AlN ceramic. Two prominent resonance modes, which are essentially responsible for the dielectric properties of the Y2O3-added AlN in terahertz regime, are characterized at omega(T01)/(2 pi) = 2.76 THz (92 cm(-1)) and omega(T02)/(2 pi) = 18.2 THz (605 cm(-1)) and are well described by the pseudo-harmonic oscillator model through theoretical fitting. The resonance omega(T01) at 2.76 THz is proposed to be due to the formation of a YAG (Y3Al5O12) secondary phase in Y2O3-added AlN ceramic. From the experimental results, good correlation is observed between the prominent peak of YAG secondary phase at 2.76 THz and thermal conductivity. Additionally, there is a high correlation between densification and refractive index of AlN ceramics fabricated by hot pressed sintering. (C) 2016 Elsevier B.V. All rights reserved.