An X-diffraction,a transmittance spectrum and absorption spectrums at different temperatures of an AIN film deposited on a(001) SiC substrate by MBE were measured on a X-diffractometer and an UV spectrophotometer(190nm～800nm) respectively. The X-diffraction spectrum shows that: there are strains in the c-axial orientation of the AIN,these sstrains are caused by the dismatching of crystal lattice constant of the AIN and the SiC substrate,and these stains will cause the stresses in the AIN film;the transmitrtance spectrum of the AIN shows that: the direct-band-gap of the AIN is about 6.2 eV;and obvious“knees”or a“shoulders”structure around 6.2 eV existing in the absorption spectrums are considered to the caused by the free excitonic absorptions in the AIN film,if not considered the formation energy of the free exciton,6.2 e is the value of the direct-band-gap of the AIN film;the“kness”or the“ shoulder”structure and position don't change with the temperature in the absorption spectrums of the AIN shows that: the temperature doesn't influence the value of the direct-band-gap of the AIN obviously,this is probable,which is caused by the stresses existing in the AIN film.