Biodegradable aliphatic-aromatic copolyesters/POSS nanocomposites were synthesized via in situ melt copolycondensation of terephthalic acid (TPA), poly(L-lactic acid) oligomer (OLLA), 1,4-butanediol (BDO) and polyhedral oligomeric silsesquioxanes (POSS) reagents (POSS-NH2 and POSS-PEG). The morphologies and dispersions of two POSS reagents in the nanocomposites and their effects on the mechanical and thermal properties were investigated. TEM and XRD characterizations confirmed that POSS-NH2 formed crystalline microaggregates and took poor dispersions in the nanocomposite, while POSS-PEG had better dispersion in the matrix. Due to the good dispersion and interfacial adhesion of POSS-PEG with the copolyester PBTL matrix, the tensile strength and the Young’s modulus greatly increased for PBTL/POSS-PEG nanocomposite. Moreover, compared with POSS-NH2 the existence of POSS-PEG imparted PBTL good flexibility and increased the mobility of the chains, so the glass-transition temperature and the heat of melting as well as the elongation at break were obviously influenced for PBTL/POSS-PEG nanocomposite.