Typical fracture morphology of Mg-3Al-1Zn alloy after cyclic deformation was investigated using optical microscope (OM), scanning electron microscopy (SEM), transmission Kikuchi diffraction (TKD) and transmission electron microscopy (TEM). Cross-section samples were extracted from secondary twin regions for TEM/TKD observation. The results showed that a large amount of {10-12}-{10-12} secondary twins were observed in the region near the fractured edge. Two-beam bright field (TBBF) technique was performed to study the types of dislocations along secondary twin boundaries. It was found that pyramidal dislocations were highly active within {10-12}-{10-12} secondary twins, which is considered to make pyramidal dislocations have the relationship with {10-12}-{10-12} secondary twins. Localized stress concentration within the secondary twins may lead to the formation of pyramidal dislocations.