The effects of external environments on the mechanical behaviour and surface micro-structure of single Kevlar fibre have been investigated. Wave diffraction theory and digital image correlation (DIC) were applied to measure the longitudinal and transverse strain of a single Kevlar fibre. The stability and credibility of transverse and longitudinal strain measurement results is guaranteed by averaging the intensity of some images and adopting an optical array extensometer respectively. The results show that the elastic modulus of a single fibre treated under UV irradiation and in water increased 30% and decreased 15% compared to that of untreated ones respectively, the surface roughness increases drastically to a maximum value and then gradually reduces to a stable value with increasing treatment time, which can provide guidance for further study of the interface performance of composite material.