Raman spectroscopy and photoluminescence (PL) measurements have been carried out to investigate the rapid thermal annealing effects on AlAs/GaAs superlattices grown on Si(100) by molecular beam epitaxy. The crystalline quality of the AlAs/GaAs superlattices annealed at 800 degrees C was remarkably improved in comparison with that of the as-grown AlAs/GaAs superlattices. The results of Raman spectroscopy for the annealed AlAs/GaAs superlattices showed that the position of the longitudinal optical phonon shifted to lower energy as the temperature increased, reaching its maximum shift at 800 OC. Above 800 degrees C, the direction of the shift reversed, moving toward higher energy with increasing temperature. This behavior is indicative of the annealing-temperature dependence of the strain. The results of the PL measurements for annealed AlAs/GaAs superlattices showed that the peak position related to the transitions from the first heavy-hole to the first electronic subband showed the same behavior. These results indicate that the crystallinity of the AlAs/GaAs superlattices is improved by thermal treatment and that the effect of the induced tensile stress is more dominant than that of the intermixing of the Ga and Al and Al due to thermal treatment.