The habit (external morphology) of a crystal is controlled by both the external (environmental) conditions of crystallization and the internal (structural) factors of the crystal. In order to separate the effects of the crystal structure and of the solvent and other external factors on the experimentally observed growth habit, the theoretical habit can be derived from the crystal structure using the periodic bond chain (PBC) theory and attachment energy considerations. According to the PBC theory the crystal habit is governed by a set of uninterrupted chains of strong bonds formed in the crystal lattice. In addition, the attachment energy (E att) is defined as the energy released per mole when a new layer is deposited on a crystal face. Since the habit of a crystal is determined by the relative growth rate (R) of the various faces, by taking R proportional to E att, the theoretical habit can thus be derived from E att. Using this approach, we obtained the theoretical crystal habit of an antitumor drug, hexamethylmelamine (HMM). The possible effect of solvents on the habit modification of HMM is discussed. This technique, based purely on the knowledge of the crystal structure, is directly applicable to other pharmaceuticals in deriving their theoretical crystal habit.