The barrier strain in Al0.22Ga0.78N/GaN heterostructure, w ith and without Si3N4 passivation layer, was investigated at temperatures from room temperature to 813K by using high resolution X-ray dif fraction. The strain relaxation occurs when the temperature exceeds 523K for the unpassivated Al0.22Ga0.78N layers. After passivation, an initial increase of the strain with increasing temperature is observed in Al0.22Ga0.78N layers, and at the higher temperatures the strain only decreases slightly in the 100-nm-thick Al0.22Ga０.78N layer, but a pronounced strain relaxation occurs in the 50-nm-thick one due to the fact that the thickness of the A10.22Ga0.78N layer is close to the critical thickness, and hence the increase of tensile strain induce d by passivation will result in partial strain relaxation via the formation of c racks or the gliding motion and multiplication of dislocations.