The three-dimensional morphology and a mechanical property of a fiber-reinforced plastic (FRP) have been investigated under uniaxial extension. A custom tensile apparatus for X-ray computerized tomography was developed for this purpose. The two FRPs used in the present study consisted of nylon 6 with glass fibers. In one of the FRPs, the fiber surfaces were treated to adhere to the nylon 6. It was observed that the fibers tended to align along the extension axis, and that cavitation occurred simultaneously during the extension. In the case of the FRP with glass fibers without any surface modification (hereafter, referred as "neat" glass fibers), void formation was dominant over the whole extension range. In the case of the FRP with surface treatment, fiber alignment occurred first and was followed by void formation. A numerical simulation was carried out to study the stress concentration around a fiber with such morphological changes during extension. Through quantitative measurements of fiber orientation and void volume, together with predictions from the numerical simulations, the effect of fiber/matrix adhesion on the morphological developments and mechanical properties of the FRPs was discussed. (C) 2017 Elsevier Ltd. All rights reserved.