The dependence of the Schottky barrier height of Ni/AlxGal - xN contact on the Al mole fraction up to x = 0.23 was studied. The barrier heights were measured by I-V, capacitance-voltage, and the internal photoemission method. The Al mole fractions were estimated from the AlGaN band gap energies measured by photoluminescence. In the range of x < 0.2 a linear relationship between the barrier height and Al mole fraction was obtained. This was consistent with the slope predicted by the Schottky rule. For x = 0.23, the measured barrier height was lower than predicted. We believed this was due to crystalline defects at the Ni/AlGaN interface.