Expanded graphite (EG) was ball-milled in a high-energy planetary-type mill under an air atmosphere. The X-ray diffraction patterns of the products show that during the milling process (up to 100 h), the out-of-plane (Lc) and in-plane (La) crystallite sizes decrease gradually from 15.4 to 11.3 nm and 24.1 to 15.5 nm, respectively. The value of Lc/La, which is used to estimate the shape of the crystallites, increases gradually from 0.64 to 0.73. Compared with most of natural graphite, this Lc decrease degree of EG is far lower. This increased value of Lc/La indicates that the crystallites of the milled EG become thicker and steeper, which is contrary to the case for natural graphite.