A well-defined styrene-acrylonitrile copolymer/montmorillonite (SAN-MMT) nanocomposite has been prepared by emulsion copolymerization of styrene and acrylonitrile in the presence of sodium ion exchanged montmorillonite (Na+-MMT). This direct and one-step polymerization technique yielded nanocomposites intercalated with styrene-acrylonitrile copolymer without occurrence of significant delamination of MMT. The purified products by hot tetrahydrofuran extraction for up to 5 days gave evidences of copolymer intercalation. Those infrared spectra obtained from the purified products revealed the characteristic absorbances due to styrene, acrylonitrile, and MMT. Room temperature powder X-ray diffraction patterns of the purified product exhibited increased 001 d-spacing about 1.60 nm. The transmission electron microscopy micrograph of unpurified products confirmed that the 1-2-nm sized silicate layers are arranged in good order. The onset temperature of purified products are found to be moved to higher temperature, while the thermograms of differential scanning calorimetry show nothing observable transition. The modulus of elasticity of the product was increased with increasing content of MMT, whereas the stress at maximum load was decreased with the increments of MMT.