The refractive index and thermo-optic property of the wide bandgap -V com-pound semiconductor of nitride Aluminum nitride (AlN) thin film has been investigatal. Theweveguide property of AlN thin film under different tempemtures is measured using a prism-thinfilm coupling device and an automatic temperature controlling wnveguide measuring device.The dependence of the refractive index of AlN on changing temperature is obtained withiteration method. The thermo-optic property of AlN thin film is measured for the first time.