Deep ultraviolet (DUV) avalanche photodetectors (APDs) based on an AlNn-SiC Schottky diode structure have been demonstrated. The device with a mesa diameter of ∼100 μm exhibits a gain of 1200 at a reverse bias voltage of -250 V or a field of about 3 MVcm. The cut-off and peak responsivity wavelengths of these APDs were 210 and 200 nm, respectively. This is the highest optical gain and shortest cut-off wavelength achieved for III-nitride based DUV APDs. It was also observed that the reverse breakdown voltage increases with decreasing device size, which suggests that the device performance is limited by the presence of dislocations. The breakdown voltage for dislocation-free AlN was deduced to be about 4.1 MVcm. The present results further demonstrate the potential of AlN as an active DUV material for future optoelectronic device applications.