Aluminum nitride thin films deposited on a low-temperature cofired ceramics substrate by reactive magnetron sputtering were investigated with regard to their crystal orientation and microstructural characteristics. Strong e-axis orientations of AIN thin films were observed when either a higher deposition temperature or an RF bias was adopted. This orientation was believed to be responsible for the high thermal conductivity of 26 W/mK for the AIN films deposited at 700 degrees C under 25-W bias. Photoluminescence spectrum in the wavelength range of 350-650 nm was analyzed to prove the involvement of potential oxygen-related defects in the thin films.