Short glass fiber reinforced composites (SGFRC) are frequently used to manufacture parts with complex surfaces by injection or compression molding. It was found that the length of fibers is often reduced during the manufacturing processes. To understand the causes of the change in fiber length, ex-situ 3D CT images are taken from tensile experiments to extract the information of the microstructure of the SGFRC, including fiber length, orientation and breakage. In the calculation of the critical length, the Kelly model is modified to include the effect of the stresses at fiber ends and the mechanical effect of internal pore defects on the composites. It is found that the microscopic interaction of short fibers breaks longer fibers, until they are shorter than the critical length. Through comparisons, it is found that the modified Kelly model is more accurate in predicting critical length of short fibers. In this study, micro-failure modes, such as fiber pulling out, interface debonding, matrix damage and pore destruction, are all characterized by micro-CT and scanning electronic microscope to analyze the complex failure mechanisms. The micro failure modes are mainly attributed to the micro geometric features and mass fraction of short fibers.