We report a simple method to prepare biodegradable polymer-amphiphile complexes by solution mixing of poly(propylene carbonate) (PPC) with octadecanoic acid (OA). The complexes were characterized by combination of thermogravimetry analysis (TGA), differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), polarizing optical microscopy (POM), transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FT-IR). Compared with the amorphous PPC copolymer, the PPC-OA-x complexes show excellent thermal stability and form thermotropic liquid crystalline state without rigid mesogenic units. The corresponding mechanism has been proposed to elucidate the observed phenomena. The stabilization effect induced by hydrogen-bonding interactions between PPC and OA molecules is responsible for the thermostability and formation of liquid crystalline state. The present findings may extend the applications of such biodegradable aliphatic polycarbonate by improving its glass transition temperature and thermoplastic processability.