Effects of two factors on the mechanical properties of clay mineral/polypropylene nanocomposites were examined. The first factor is the presence of a small amount of poly(diacetone acrylamide) formed between clay mineral layers. This material was expected to separate effectively the stacked layer structure of the clay mineral on mixing with polypropylene. The stacked layer structure, however, was not separated sufficiently in spite of expansion of the interlayer distance of clay mineral, leading to poor improvement of the mechanical properties of the nanocomposite. Another factor is the kind of clay mineral. Montmorillonite and mica resulted in less favorable separation of the stacked structure than hectorite, but the resulting nanocomposites gave outstanding improvement of the properties. The stiffness of clay mineral layer was considered to influence the properties of the nanocomposite strongly.