Hexagonal nanoplatelets of layered double hydroxide containing Mg2+ and Al3+ cations and CO32- anion (MgAl-CO3-LDH) were successfully synthesized via an ethylenediamine-assisted hydrothermal route. The obtained samples were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectrometry, scanning electron microscopy (SEM), transmission electron microscope (TEM), and UV-vis diffuse reflectance spectrum, respectively. The results show that ethylenediamine not only inhibits impurity phases generated in the synthesis of MgAl-CO3-LDH but also can effectively control the crystal size of MgAl-CO3-LDH in the range of 50-100 nm. In particular, a possible formation mechanism of hexagonal nanoplatelet MgAl-CO3-LDH is proposed. The present work develops a simple and mild route to crystallize MgAl-CO3-LDH with hexagonal nanoplatelet morphology, which is a key engineering step toward its potential size-dependent applications. (C) 2010 Elsevier B.V. All rights reserved.