Spatial and temporal variations of illite crystallinity (KI,Full width half maximum of illite diffraction peak at 1. Onm) were investigated on the <2mum of samples from three loess sections since the last interglacial periods across the Chinese Loess Plateau, using the X-Ray diffraction (XRD). Results indicate that KI values in the Changwu (CW; 35°12'N, 107°48'E) and Weinan (WN; 34°18'N,109°30'E) sections is higher in the paleosols (S_0 and S_1) than in the loess units (L_1), showing loess-paleosol alternation, similar to the pattern of magnetic susceptibility (MS). In the Huanxian section (HX ; 36° 37' N,107°19'E), the contrast in the KI values between paleosols (S_0 and S_1) and loess units is less, but bottom of paleosol S_1 still has higher ratios than other units. Spatially, the mean KI values of each of the stratigraphic units (S_0 and S_1) generally display a southeastward increase along the NW-SE sections, similar to the pattern of MS. In the loess units (L_1), the mean KI values between Huanxian and Changwu sections show less difference and are rather lower, but the higher mean KI values exit in the Weinan Section. Results also show changes of the KI values in the three sections could not affected substantially by the dust provenance, eolian sorting and sedimentation rate. Since spatial changes in the KI values are overall similar to the present day climate and the past climates,we therefore those changes of the KI values are primarily caused by in situ pedogenesis and chemical weathering, and these propose changes reliably document changes in the intensity of the East Asian summer monsoon. Relatively, the KI values are not more sensitive to climate change than other summer monsoon indexes, such as MS, Rb/Sr and Fed/Fet. The KI values of the paleosol S_0,S_1 in the Weinan and Changwu sections, and the loess L_2 in the Weinan section could effectively recorded the changes of East Asian summer monsoon. In those units, loess L1 of the Weinan section is the weakest developed unit, with the MS values of 125. 3x 10~(-8)m~3kg~(-1). Tentative estimates for the unit yield a mean annual precipitation of about 586mm based on the climofunction for paleorainfal from Song et al. (2012, 2013). This result indicates that the precipitations for the changes of KI values in Weinan L_1 unit are similarity with that of the modern pedology.