Engineering plastics and their composites are utilized in mechanical and civil engineering structures. It is very important to understand the fatigue crack growth mechanism of polymeric materials for structures from the viewpoint of long-term reliability. The aim of this study is to investigate fatigue crack growth properties and the fracture toughness of polytrimethylene terephthalate (PTT) moldings under different thermal treatments. Two types of samples of the same crystallinity with different crystal structure were formed from the same moldings by experiencing different thermal history : one was a quenched sample (QS), the other was an isothermal crystallization sample (IS). The QS had small crystal structure, while the IS sample was composed of spherulites with band structure. The fracture toughness of the QS was 80% higher than that of the IS before the annealing treatment. Both samples showed similar fatigue crack resistance, which was much lower than the fracture toughness. The annealing treatment increased the degree of crystallinity and reduced the fracture toughness and fatigue properties of both samples because the plastic deformation was limited. The fatigue crack resistance of the IS was higher than that of the QS. Fractographic observation revealed that the band structure in the spherulites provided resistance to crack growth.