Abstract: To fully understand the underlying toughening and reinforcing effect of short aramid fibers to interfacial and structural properties of composite sandwich structures, experimental studies were carried out for investigating toughening effects and mechanisms of aramid fiber interfacial toughening on a sandwich beam, consisting of an foam aluminum covered with two carbon fiber/epoxy composite surface layers. Low-density short aramid fiber films were inserted at the interface during the sandwich fabrication process, to improve the interfacial bonding properties through the bridging effect of short fibers. In-plane compression tests were carried out to investigate the effect of short aramid fibers on structural properties and failure mode of sandwich beam. The critical energy release rates of face-core interface with various toughening parameters were measured under asymmetric double cantilever beam (ADCB) condition. According to the results, the in-plane compression strength of sandwich beams and critical energy release rates between face sheet and core were all improved for all specimens toughened with short Kevlar fibers, especially for specimens toughened with hybrid-length short Kevlar fibers. The interfacial toughening performance and underlying mechanisms for short aramid fiber toughening were discussed and analyzed using SEM.