We report on the blueshift of energy gap and the nitrogen rearrangement resulting from the thermal treatment of GaAsSbN samples. The composition ranges of N and Sb are from 2% to 3% and from 7% to 11%, respectively. By comparing the photoluminescence peak energy and absorption energy gap, we found that the thermal annealing leads to compositional homogenization. When the annealing temperature is higher than 750 °C, the local vibration mode (LVM) absorbance of isolated substitutional nitrogen NAs starts to increase with the temperature. This finding supports the argument that nitrogen pairs thermally dissociate to isolated nitrogen. In additions, the increase of LVM absorbance accompanies the blueshift of energy gap, suggesting that the blueshift is the consequence of the nitrogen rearrangement. The decomposition of nitrogen pairs has also been observed by photoreflectance spectroscopy (PR). Multiple peaks were resolved from the PR spectra of as-grown GaAsSbN. They are relevant to transitions from regions with different nitrogen pairings and isolated nitrogen. As the annealing temperature increases, the peaks gradually merge to that of isolated nitrogen, indicating the decomposition of nitrogen pairings.