The thermal diffusion method is employed for ZnO loading into ZSM-5 crystals (ca. 120 μm along c axis) which is regarded as a good host material for guest encapsulation. Optical microscopy photographs and X-ray powder diffraction spectra are obtained and show that the ZnO clusters are infiltrated into ZSM-5 crystals, instead of locating on the surface of the crystals. An intense porosity analysis based on density functional theory is used to illustrate the pore texture of the ZnO-loaded samples. The sample prepared at 650 °C has ZnO clusters existing mainly in the external surface of ZSM-5. As for the sample prepared at 750 °C, the micropores of ZSM-5 have been occupied.