Abstract: Interstratified illite-montmorillonite series occur universally in claystones and in sandstones as fillings. The types and proportions of interstratified illite-montmorillonite series are believed to be very important in the interpretation on clastic diagenesis. For this reason, the present paper has documented the importance of identification and compositional determination of interstratified illitemontmorillonite series by x-ray diffraction analysis, and clearfied some confused ideas, including the differences between regularly and irregularly interstratified illite-montmorillonite series, and between montmorillonite and irregularly interstratified illite-montmorillonite as well as estimation of their proportions.