Optical properties of GaN/Al0.2Ga0.8N superlattices have been investigated by Raman scattering and photoluminescence measurements. It is found that the A1(LO) phonon decreases in frequency with decreasing quantum-well width. The frequency shift is attributed to the effect of phonon zone folding. Through the study of photoluminescence, we show that our observation of the zone-folding effect on optical phonon in GaN/Al0.2Ga0.8N superlattices is due to the sharpness of the interfaces between barrier and well layers. The sharp interfaces prevent the appearance of mixed interface modes which mask the phonon effect of zone folding in previous reports.