2, 4, 6, 8, 10, 12-Hexanitrohexaazaisowurtzitane(HNIW, or CL-20) exists in four crystalline polymorphs(α-, β-, γ-and ε-) which can be transited in different solvents at ambient conditions. In this work, the ε-HNIW crystallization process in a saturated HNIW solution in ethyl acetate being added to n-heptane with different addition rates. Crystal polymorph and morphology of HNIW were studied by Fourier transform infrare spectroscopy(FTIR) and scanning electron microscope(SEM), respectively. It is revealed that crystallization mechanism of ε-HNIW is kinetic-controlled when n-heptane addition rate is above 100 mL·s^-1 and the crystallization process of HNIW is that metastable phase β-HNIW nucleus firstly formed and growed; then, β→ε phase transition occured and stable phase ε-HNIW increased; at last, β-HNIW disappeared and ε-HNIW attained completely. When n-heptane addition rate is below 20 mL·s^-1, crystallization mechanism of ε-HNIW is thermodynamic-controlled and only ε-HNIW nucleus formed and growed without phase transition in the whole crystallization process. The hydrate α-HNIW will be formed if water existed in HNIW solution. So, the non-solvents used for preparation of ε-HNIW should be non-polar or poor-polar solvents, not solvents with high values of dipole moment.