Based on the analysed result, there established a curve relationship between the receiving slit (Rs) of a diffractometer and FWHM, i. e. FWHM = ARs2 BRs C. The methods based on Wang's formulae (1994) to evaluate the error levels caused by the scanning speed and step size in FWHM and maximum intensity of an XRD peak were described. These errors induced by the step size and scanning speed should be below 5 %. Reviews of sample preparation, X-ray diffraction setting and the role played by interlaboratory IC standards were given, involving (1) determining sample geometric conditions, choosing optimum instrumental settings and using interlaboratory illite crystallinity standards are the most important procedures in the measurement of illite crystallinity; (2) the IC value from the same sample will vary with the changes of such geometric conditions and instrumental settings; (3) IC values without correction for the IC standards (Kisch, 1991, Warr et al. , 1994) cannot be comparable with each other and (4) this will mislead the research when the boundaries of the anchizone are derived from these uncorrected IC values.