Glass beads, non-modified and modified with coupling agent, were filled separately into high density polyethylene to obtain composite materials with different interfacial adhesion strengths. In situ tensile tests reveal the damage mechanisms, which are mainly induced by the interfacial debonding. The interfacial debonding process is observed and studied. The debonding stress is found to be linearly related to the opening angle formed at two poles of the particles. Initial and final opening angles, in addition to the corresponding debonding stresses, are measured. The interfacial fracture energy obtained by using the Griffith fracture theory is found to be 0.028 J m(-2) and 0.058 J m(-2) for mechanical anchorage and physical entanglement across the interface, respectively. The stronger the interfacial adhesion, the smaller is the maximum opening angle and greater the debonding stress.