Clay mineralogy of paleosols provides more information about Quaternary climatic changes and paleoenvironmental conditions. Clay minerals of an alluvial fan in eastern Isfahan were studied, using x-ray diffraction analysis. Studied paleosols are located on old Qf1 and Qf2 geomorphic surfaces. Qf1 geomorphic surface is older and topographically higher than Qf2. The results of clay mineralogy show dominance of palygorskite and smectite in calcic (Bk) and argillic (Bt) horizons of Qf1 geomorphic surface, respectively. Calcrete (Bkm) in the paleosol of the Qf1 surface was formed in the semi-arid paleoclimate. It seems that during more humid periods, carbonates enhancement and porosity decreases in calcrete (Bkm) horizon have caused increase in ionic activity and formation of pedogenic smectites in this horizon. In the next drier periods, the calcrete (Bkm) horizon was enriched in palygorskite via translocation and autogenic formation. The palygorskite fibers in calcrete (Bkm) have been protected by calcite crystals. The comparison of the argillic horizons of Qf1and Qf2 geomorphic surfaces indicates dominance of smectite in Qf1 argillic (Bt) horizon and palygorskite in Qf2 argillic-calcic (Btk) horizon. Regarding to Uniformity of parent material and landform, this difference is due to high weathering in the argillic (Bt) horizon of Qf1 and protection of palygorskite in the argillic-calcic (Btk) horizons in the soil of Qf2 surface. It is recommended to use clay mineralogy of calcic horizons, argillic horizons and calcretes as a powerful tool in geomorphology investigations.