Up to 15 nm blueshift in the low-temperature photoluminescence (LT-PL) peak wavelength of Si-doped In0.2Ga0.8N/GaN single quantum well (SQW) is reported after rapid thermal annealing in atmospheric N-2 ambient at temperatures below 1000 degrees C. We found that the thermal annealing not only blueshifts the LT-PL peak wavelength but it also improves the optical properties in terms of an increase in LT-PL peak intensity and a reduction in the spectrum width. Under the same thermal annealing conditions, however, a dielectric encapsulant layer (a spin-on silica layer) suppresses the blueshift of the LT-PL peak wavelength of the In0.2Ga0.8N/GaN QW. The effect of the thermal annealing temperature and time on the blueshift of the LT-PL peak wavelength is also reported. The possible mechanism responsible for the thermal annealing effect on the band gap energy of the In0.2Ga0.8N/GaN SQW is discussed.