The effects of ionic size in octahedral and two kinds of tetrahedral sites (M, T1, and T2) on the thermal expansion of cordierite were systematically investigated by means of molecular dynamics simulations. Thermal expansion along the a axis could be controlled by varying the ionic size in the M-site, and expansion along the c axis could be controlled by varying the ionic size in the M- and T2-sites. However, the ionic sizes in the M-and T2-sites had opposite effects.