The mechanical and thermal stability of low‐angle boundaries in zinc single crystals was studied by methods combining metallographic techniques with x‐ray reflection microscopy and diffraction analysis. It was found that low‐angle boundaries perpendicular to the basal plane do not offer any appreciable resistance to a shearing force parallel to the basal plane if they are pure tilt boundaries, but are substantial barriers if they are asymmetrical. If the external force exceeds a critical value, these asymmetrical low‐angle boundaries become sites where fracture may occur. The strengthening effect of the asymmetrical low‐angle boundaries is attributed to the effective stress field of the screw dislocations, which are mechanically stabilized by the edge dislocations. The stress field of the screw dislocations is also responsible for the thermal instability of the asymmetrical low‐angle boundaries, which on annealing are gradually converted into pure tilt boundaries. A coarse substructure established after annealing is thermally very stable. On solidification after partial melting of the crystal, the coarse substructure is re‐established, whereas the fine substructure is entirely modified.