To improve the beta-phase content of polyvinylidene fluoride (PVDF), diacetylene (DA) monomers were embedded in PVDF nanofibers. The PVDF/DA nanofiber membranes were fabricated using the electrospinning technique and polymerized to form polydiacetylene (PDA) after UV irradiation. In this study, the effect of PDA on the crystalline structure of PVDF nanofibers was examined and compared with cast films. The nanofiber morphology was assessed by SEM, while Fourier transform infrared spectroscopy and wide-angle X-ray diffraction were performed to characterize the crystallinity behavior. The mechanical properties and crystal polymorphism of the electrospun PVDF/DA nanofibers were also studied before irradiation, after irradiation and after colorimetric transition induced by the heat treatment. The results showed that the addition of PDA can effectively improve beta-phase crystalline PVDF in both the as-cast film and the electrospun nanofibers which is greater in the latter. The electrospun PVDF/DA nanofibers after UV irradiation exhibit greater intensity of beta-phase in comparison with those before irradiation. Furthermore, the fraction of beta-phase decreased when the blue phase of the PVDF-PDA nanofibers undergoes a heat-induced thermochromic transition to a red phase. The tensile strength and elongation at break as well as the tensile modulus were notably improved after heat treatment of the electrospun PVDF-PDA nanofibers.