The thermal and impact behavior of poly(butylene terephthalate) (PBT) and its composites toughened with E-glass fiber were investigated at various testing temperatures. The crystallization rate of PBT composites was faster than that of pure PBT due to the nucleation agent role of glass fiber in the PBT melt, and the degree of crystallinity increased with glass fiber content regardless of testing temperatures. Notched Izod impact strength increased more rapidly at 1$25^{circ}C$ and 175$^{circ}C$ than at $25^{circ}C$ and 75$^{circ}C$. The fracture morphology and subsurface structure of specimens fractured by impact test were observed using SEM (Scanning Electron Microscope) and POM (Polarized Optical Microscope) . And the results were utilized to illuminate the relationship between morphology and impact properties. By the morphological analyses, plastic deformation in matrix, interfacial debonding phenomenon between matrix and glass fiber, and cavities that occurred by pulling out glass fiber from the matrix were observed on the fractured surfaces.h