A laboratory X-ray computed tomography (XCT) device was used for the quantitative 3D characterization of damage mechanisms in short glass fiber reinforced polymers. Geometry for test specimens was developed that allows for high resolution and tensile testing within the XCT device. Interrupted in-situ tensile tests on specimens with perpendicular main fiber orientations were performed. Fiber orientation and damage mechanisms were analyzed on single fiber basis using XCT data analysis tools. Damage mechanisms that were identified are fiber pull-out, fiber breakage, detachment and matrix cracks. Depending on the main fiber orientation pull-out or detachment is predominant. The determined fiber orientations for different regions showed a correlation to the region of final fracture.