In order to investigate the effect of annealing on the crystalline stucture of poly(butylene terephthalate)(PBT) and the selective degradation of PBT by amines, PBT filaments(FDY) were annealed under various condition, thereafter crystallinity, birefringence, and tensile properties were measured, also the aminolysis behavior of annealed PBT was observed. The plateau region in load-extension curve indicates not only crystal transition of $alpha$longrightarrow$eta$, but also orientation behavior of crystalline phase. Additionally, the "maximum extension" region, which might stand for the crystalline morphology, was newly defined. There was a linear relationship between "maximum extension" and crystallinity. The selective degradation action of amine to PBT was also observed and proved by the measurements of density and intrinsic viscosity as in the case of the aminolysis of PET. And "solvent-induced crystallization" was also brought about during the selective degradation of PBT filaments by 40% methylamine solution.s by 40% methylamine solution.