The effect of external loading of fiber on fiber/resin interfacial adhesion has been studied using the microbond technique. The results show that the applied load on the fiber weakened the fiber/matrix bonding at the interface. The effect of cyclic loading of fiber on fiber/matrix interfacial adhesion was also investigated by loading and unloading the fiber by means of a moving platform oscillating vertically at a specific frequency. After the cyclic loading process, residual bond strengths were measured using the TRI microbond pullout procedure. Fiber/matrix combinations of Kevlar 49(R)/Epon 828(R) and Kevlar 49/polycarbonate have been investigated using this technique. Significant bond strength reduction is observed when load is applied and after cyclic loading. In the latter case, most of the loss occurs after relatively few cycles. Possible bond weakening mechanisms are proposed to explain the experimental results.