Sphere-like mesoporous CdS nanocrystals with high crystallinity and big surface area were successfully fabricated by ultrasound irradiation at room temperature. The as-synthesized CdS with and without ultrasound irra diation was investigated by the characterizations of X-ray diffraction(XRD), scanning electron microscopy(SEM), transmission electron microscopy(TEM), and Brunauer-Emmett-Teller(BET) surface areas. The photocatalytic acti vity of the CdS was evaluated by photocatalytic degradation of methyl orange under visible light(λ>420 nm) irradia tion. A possible mechanism for the formation of the CdS nanocrystals with mesoporous structure under ultrasound irradiation was proposed. The results show that both the template role of the triblock copolymer of P123 and the effect of ultrasound-induced aggregation are mainly responsible for the formation of mesoporous structure. On the other hand, the energy generated from acoustic cavitation can effectively accelerate the crystallization process of the amorphous CdS. Sphere-like mesoporous CdS nanocrystals with high crystallinity and big surface area were successfully fabricated by ultrasound irradiation at room temperature. The as-synthesized CdS with and without ultrasound irra diation was investigated by the characterizations of X-ray diffraction (XRD), scanning electron The photocatalytic acti vity of the CdS was evaluated by photocatalytic degradation of methyl orange under visible light (λ> 420 nm) irradia tion (SEM), transmission electron microscopy (TEM), and Brunauer-Emmett- Teller . A possible mechanism for the formation of the CdS nanocrystals with mesoporous structure under ultrasound irradiation was. The results show that both both template role of the triblock copolymer of P123 and the effect of ultrasound-induced aggregation are mainly responsible for the formation of mesoporous On the other hand, the energy generated from acoustic cavitation can effectively accelerate the crystallization p rocess of the amorphous CdS.