In general, wurtzite CdS exhibits a higher photocatalytic activity than zinc-blende CdS. However, the underlying physicochemical reasons responsible for the differences of photocatalytic activity between the wurtzite and zinc-blende CdS are still unclear. In this work, the structural characteristics, band structures, density of states, bond populations, optical properties and charge carrier effective mass of wurtzite and zinc-blende CdS were investigated based on first-principle theoretical calculations. The calculated results indicate that the distortion of CdS4 tetrahedron units results in the formation of internal electric field in wurtzite CdS, which is beneficial for the efficient separation and diffusion of photogenerated charge carriers. Contrarily, the internal electric field is absent in zinc-blende CdS. Moreover, the effective masses of photogenerated charge carriers of wurtzite CdS are smaller than those of zinc-blende CdS, implying faster migration of photogenerated charge carriers to perform photocatalytic reactions on wurtzite CdS surfaces. All the above factors result in the lower recombination rate of photogenerated charge carriers within wurtzite CdS. Therefore, it is not surprising that wurtzite CdS usually shows a higher photocatalytic activity than zinc-blende CdS. This investigation will provide some new understanding on the difference of photocatalytic activity between wurtzite and zinc-blende CdS. (C) 2016 Elsevier B.V. All rights reserved.