A seed-mediated growth method is described for the synthesis of a novel type of Au nanoparticles-truncated decahedrons with a pentagonal, platelike structure (see figure). The precursor is based on the AuCl/oleylamine complex. TEM analyses reveal that the truncated Au nanoparticle has two types of truncation: one is parallel to the five-fold axis and the other is perpendicular to the five-fold axis.