We investigated the mechanisms of improved GaInNAs optical properties by thermal annealing. The absorption spectra measured for the bulk layer indicated that the large shift in the PL wavelength was probably caused by a bandgap shift in the GaInNAs itself. The cathodeluminescence measurements revealed that the enhancement of the PL intensity was generated by uniform emission from the entire region; in comparison, nonuniform dot-like regions exist in an as-grown GaInNAs layer. These analyses, which is peculiar to this type of material system, should be helpful for further improving the crystal quality, thus helping to enable semiconductor lasers with excellent high-temperature performance.