To reveal palaeoclimatic evolution and aridification events of Early Oligocene-Early Miocene period on the northeast margin of Tibetan Plateau, the authors investigated micro-morphology, relative content of clay minerals and V(Ill+Chl)/V(Sme) ratios of sediments in Lanzhou Basin by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). The relative content of clay minerals shows that the sediments are composed of illite, smectite, chlorite and minor illite-smectite mixed-layer clay (Ill/Sme), indicating that aridificati on prevailed in Lanzhou in that period, with intervals of relatively warm and humid climate conditions. According to relative content and V(Ill+Chl)/Ill(Sme) ratios, the palaeoclimate evolution in Lanzhou could be divided into four stages, i.e., (Ⅰ) relatively warm and dry and slightly humid period (31.5～28.8 Ma), (Ⅱ) relatively cold and dry period (28.8～26.2 Ma), (Ⅲ) relatively warm and humid period (26.2～22.1 Ma), and (Ⅳ) relatively warm and dry and slightly humid period (22.1～16.5 Ma). SEM results indicate that clay minerals were apparently corroded in the relatively warm and humid period and presented distinctly physical weathering characteristics in the relatively arid period. The aridification events probably resulted from the multi-stage uplifts of the Tibetan Plateau.