This study investigates the effect of post-curing temperature (80, 100, 120, 140, and 150 °C) on the flexural properties and fracture morphology of unsaturated polyester resin (UPR) composites. The results indicate that post-curing at temperature 80 °C performed the lowest strength (46 MPa) and modulus (0.75 GPa) due to incomplete polymerization, which was characterized by the presence of voids in SEM observations. The mechanical properties reached their optimum values at 140 °C, with a flexural strength of 114 MPa and a modulus of 2.65 GPa. This was supported by a dense and smooth fracture morphology, indicating a high cross-linking density. However, at 150 °C, over-curing occurred, triggering thermal degradation in the form of polymer chain scission and micro-cracks, which led to a drastic decrease in strength to 32 MPa and modulus to 0.70 GPa. Thus, it is concluded that the optimum post-curing condition to maximize the flexural properties of UPR in this study is 140 °C for 60 min.