A gas anti-solvent process (GAS) was used to recrystallize cyclotetramethylenetetranitramine (HMX). A series of experiments were carried out, changing key operating parameters such as the solvent type, temperature, CO(2) addition rate, initial solute concentration, and agitation rate. It was observed that the crystal structure and the crystal phase of precipitated ultra fine HMX particles depended on the solvent type used in the GAS process. As the effects of each parameter on particle size were variable, small sized HMX particles could be obtained by the GAS process at low temperature, high flow rate of CO(2), low solution concentration, and high agitation rate. DSC analysis showed that small sized particles of HMX were expected to have high performance of pyrolysis due to its low activation energy (E(a)) and large enthalpy change (Delta H) at the decomposition of HMX. (C) 2011 Elsevier B.V. All rights reserved.