The thermal properties of the SMPU-OOBAm composites were investigated by TGA and DSC. The TG curves show that all samples appear to decompose in two stages, which corresponds to the decomposition of the soft (at 200–300 °C) and hard segments (at 350–450 °C) of the SMPU matrix (see Figure 4a). The TG results also show that the decomposition temperatures of the SMPU-OOBAm composites increase when the OOBA-content increases, which implies that the decomposition of SMPU-OOBAm is restrained when OOBA is incorporated. However, when the OOBA-content increases, the decomposition quantity in the first stage increases because of the increased decomposition of OOBA. Additionally, a broad weak peak is observed at about 210 °C on the DTG curves. This result demonstrates that the thermal stability of the composites at the first stage improves (see Figure 4b). The improved thermal stability make the composites good polymeric materials.