This article deals with the improvement of flame retardancy of polybutylene succinate (PBS) using isosorbide modified by sulfur, silicon or phosphorus as fire retardants (FRs). The syntheses of these FRs are detailed, and all molecules were analyzed by liquid state nuclear magnetic resonance (NMR). Prior to be incorporated into PBS, the thermal stability of all flame-retardants was analyzed by thermogravimetric analysis (TGA). The results showed that all FRs exhibit thermal stabilities (>170 °C) suitable for the processing of PBS (140 °C). The fire properties of the PBS formulations were investigated and it was shown that i) sulfur modified FRs showed no improvement of PBS flame retardancy, ii) silicon modified FRs showed slight improvement acting as char forming agent when blending with intumescent FR, iii) performances of phosphorus modified FRs depend on the oxidation degree of phosphorus when evaluated by cone calorimetry, and at UL-94 tests these phosphorus modified FRs achieved V-0 rating with dripping without ignition of the cotton.