A new optically active poly(ester-imide) (PEI) was synthesized from the interfacial polymerization of Ethyl l-lysine-N,N'-ditrimellitoyl diacyl chloride with bis phenol A. Ultrasonic irradiation was applied to compose nanocomposites from the synthetic polymer and different ratios of organo-modified montmorillonite (MMT). The formation of PEI was confirmed by (1)H NMR, FT-IR spectroscopy, specific rotation, and elemental analysis. The resulting nanocomposites were characterized by FT-IR, powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and thermogravimetric analysis (TGA). The results indicated that the composites were dispersed homogeneously in PEI matrix on nanoscale. An improvement in heat stability is indicated from TGA analysis.