For understanding the crystallization behaviors of the hard phase in the SMPU-OOBAm composites, the isothermal crystallization kinetics were investigated using DSC analysis. Detailed calculations are provided in Section 2.5. The crystallization temperature was fixed at 137 °C in this experiment. Figure 6a shows that the exothermic peak of SMPU-OOBAm widens when the OOBA-content increases during the isothermal crystallization process, which suggests the increased crystallization time. According to Equation (1), the relationships between relative crystallinity (Xt) and time are shown in Figure 6b. The curves exhibit an S pattern. The crystallization rate of the hard phase is slow during the initial stage and during the later period of crystallization. Moreover, the slope of the curves decreases when the OOBA-content increases. Thus, the crystallization rate of the hard phase decreases when the OOBA-content increases. The Avrami equation (Equation (2)) was used to denote the complete crystallization behaviours of the SMPU-OOBAm composites. Generally, and for convenient analysis, the Avrami equation is converted to Equation (3) via linearization. Figure 6c presents the dependence of log{−ln(1 − X(t))} on log(t). It was found that log{−ln(1 − X(t))} is linearly related to log(t), but the curve slightly deviates during the later period of crystallization. This deviation implies the existence of a secondary crystallization, which should be attributed to the phase transformation from sphero-crystal to lamella-crystal. Finally, based on the original data and fitting a straight line, the kinetic parameters n, K, and t1/2 of the isothermal crystallization were easily calculated. The results are summarized in Table 2. The n value of all samples is ~1.3, which further indicates that the crystallization mechanism of the hard segment of SMPU in all composites is the nucleated mechanism. The crystallization rate decreases because the K value decreases when the OOBA-content increases. One possible reason for this is that the increased OOBA content might affect the aggregation of the hard segment of SMPU, though they provide more crystal seeds.