Three dimensional (3D) flower-like BiOCl nanostructures were synthesized by a facile and mild solution chemistry method at room temperature without any post-treatment. It is found that the volume ratios of ethylene glycol to water (EG-W) play an important role in the nucleation and growth of BiOC1, which consequently determine the self-assembly and thus the morphology of BiOCl nanostructures. In the solution of high EG-W ratio, the thin nanosheets self-assemble into 3D flower-like nanostructures, resulting in large surface area and abundant oxygen vacancies. The oxygen vacancies facilitate the trapping and transfer of photogenerated electrons to adsorbed oxygen molecules, producing active O-2(center dot)- species. As a result, the BiOCl nanoflowers exhibit much higher photocatalytic activity for RhB degradation under visible light irradiation than their nanosheets counterparts synthesized in the solution of low EG-W ratios.