The fine structure and provenance of clays across the Permian-Triassic boundary (PTB) in the Yanlou section were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). The results suggest that the layers have two different provenances. The sample of YN06 exhibits a condensed texture with an oriented arrangement of detrital clay particles and consists mainly of illite and minor chlorite, with irregular outlines or ragged edges. The dehydroxylation temperature occurred at 590℃, indicating that clay minerals are composed of trans-vacant (tv) octahedral sheets derived from weathering of detrital illite. Other samples are composed mainly of I-S, illite and minor smecrite, chlorite, with dehydroxylation temperature all above 600 ℃, suggesting that clay minerals consist of a mixture of tv and cv sheets and were derived from a mixture of terrigenous and volcanic sources.