Investigating the dependence of the catalysis on the size and structure of materials is of great significance for exploiting catalysts with characteristics of high activity, low cost, and new property. Non-precious metal catalysts bear high hope to meet the increasing demands of industrial applications in a cost-effective and environmentally friendly way. In this work, we take size-controlled BiOCl nanosheets as examples, which are synthesized via a hydrothermal method by changing the reaction conditions. The BiOCl nanosheets were characterized in details to understand their size-property relationships, and were found to exhibit a series of thickness-dependent physicochemical properties, including specific surface area, light absorption, and the separation efficiency of photo-generated charge carriers. Moreover, this work demonstrates the first example that BiOCl nanostructures have very high catalytic activity for the reduction of nitrophenols by sodium borohydride, without any light irradiation. The high catalytic activity of BiOCl nanosheets was proved to be due to the metallic Bi-0 clusters that were produced by surface Bi (III) reduction. The catalytic activity increased greatly with a decrease in the average thickness from 106.42 nm of BiOCl(H2O) to 3.47 nm of ultrathin BiOCl, because the increased specific surface area provided more active sites for catalytic reactions. As a result, this work provides evidences for the size property relationships of nanostructured catalysts as well as some inspirations for exploiting novel heterogeneous catalysis of BiOCl nanomaterials. (C) 2017 Elsevier Inc. All rights reserved.