Abstract: AIMTo investigate the lattice mechanisms involved in the increased dissolution effect of polyethylene glycol (PEG 6 000) dispersion system on poorly soluble drug silymarin (SILY). METHODSFusion method was used to prepare the solid dispersions of SILY with PEG 6 000. Evaluation of the improvement of dissolution was performed with dissolution studies in vitro. X-ray powder diffraction combined with diffraction peak pattern-fitting procedure were applied to quantitatively analyze the changes of lattice parameters. The interaction of SILY and PEG 6 000 was also determined with Fourier transform-infrared (FT-IR) spectroscopy. RESULTSThe dissolution rate of SILY was considerably increased when formulated in solid dispersion of PEG 6 000 as compared to pure SILY. The datum from the X-ray diffraction showed the changes in the lattic spacings and particular diffraction peaks (position and the intensity) of PEG 6 000 and SILY. These could explain the increased rate of SILY released from solid dispersion system. The information of FT-IR spectroscopy showed the absence of well-defined drug-polymer interaction. CONCLUSION The dissolution improvement of poorly soluble SILY from solid dispersion of PEG 6 000 can be illuminated by the changes of the lattice parameters of PEG 6 000 and the drug.