Biopolymer nanocomposite of polylactic acid was synthesized from lactic acid using tin ions exchanged Cloisite Ca++. The tin molecules between the Cloisite Ca++ layers catalyzed the lactic acid polymerization and also this clay played a role as a nanofiller. Presence of tin molecules increased the molecular weight of the polymer compared with the polymer that was synthesized with the presence of just Cloisite Ca++. Both exfoliated and intercalated nanocomposite structures were observed as the results of X-ray diffraction analysis and transmission electron microscopy images. Also, changes of polymer's thermal degradation and other thermal properties such as glass transition temperature, melting temperatures were investigated via thermogravimetric analysis and differential scanning calorimetry.