The crystalline morphologies of PBT (poly butylene terephthalate) and its glass fiber reinforced composite systems were investigated in a thin-film form by polarized optical microscopy and wide-angle X-ray diffraction. Three different types of PBT morphology were identified in the Maltese cross pattern: 45degrees cross pattern (usual type) by solvent crystallization, 90degrees cross pattern (unusual type) by melt crystallization at low crystallization temperature, and mixed type by melt crystallization at crystallizalion temperatures higher than 160degreesC. The glass fibers increased the number density of spherulites and decreased the size of crystallites acting as crystallization nucleation sites without exhibiting trans-crystallinity at the vicinity of the glass fiber surfaces. Finally, the storage modulus was analyzed by using a dual-phase continuity model describing the modulus by the power-law sum of the amorphous- and crystalline-phase moduli. The crystalline-phase modulus was extracted out from the PBT polymer and composite systems containing different amount of crystallinity.