New nanocomposites of poly(butylene succinate-co-ethylene terephthalate)/nano hydroxyapatite were synthesized using two-step in situ polycondensation. The composition, microstructure, morphology and dispersion of nanoparticles in the nanocomposites were studied using proton nuclear magnetic resonance (1H NMR), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that presence of nano hydroxyapatite catalyzes the reaction and there is a chemical bond between nanoparticles and polymer which leads to a good particle dispersion but it doesn’t affect molecular sequence length. Nanocomposites’ thermal properties evaluated by differential scanning calorimetry (DSC) and X-ray diffraction (XRD), showed that crystallinity and crystallite size slightly decrease with the nanoparticles weight fraction. Moreover, the elastic modulus slightly increases and tensile strength and elongation at break decrease with the nanoparticles weight fraction according to dynamic mechanical thermal analysis (DMTA) and tensile analysis. Introducing nano hydroxyapatite increases the hydrolytic degradability dramatically because of the presence of hydrophilic nanoparticle and lower crystallite size.