In the present study, monodispersed colloidal zinc oxide (ZnO) nanospheres with high uniformity were successfully synthesized via a newly developed two-stage solution route, designated the seed solution method, based on a polyol-mediated preparation approach. The narrow size distribution of the synthesized ZnO nanospheres can be precisely controlled in sizes ranging from 110 nm through 330 nm during the second stage of the synthesis by varying the amount of seed solution added. The structure, composition, morphology, size, and size distribution of the as-synthesized ZnO nanospheres were characterized. The results of our experiments suggest that a core-shell growth mechanism was employed for the first time to form ZnO nanospheres. Furthermore, the as-obtained porous ZnO products, which were calcinated at 400 A degrees C, were assessed as an efficient heterogeneous photocatalyst for photocatalytic degradation of methylene blue (MB) aqueous solution under UV-light irradiation at room temperature. Interestingly, the calcinated ZnO products exhibited photocatalytic activity that was significantly enhanced in comparison with that of uncalcinated ZnO. Moreover, particle size had a crucial effect on degradation efficiency. Reducing the particle size improved the efficiency of photocatalytic MB degradation, indicating that the photocatalytic reaction of MB on the ZnO product was a structure-sensitive reaction. The as-obtained ZnO products were easily separated from waste solutions and reused, demonstrating their significant potential as photocatalysts in environmental remediation applications.