The tensile fracture morphology and longitudinal surface structure of the AZ80 magnesium alloy treated by solid solution and ageing methods were observed by FFSEM （field emission scanning electron microscope） and OM（optical microscope）. The results indicate that the fracture morphology of the as-cast AZ80 magnesium alloy is characterized by apparently intergranular brittle fracture

which is mainly created in the boundary where distributed amounts of β-Mg17Al12 phase exist as net structure. After solid solution treatment

the average tensile strength and elongation of the alloy are remarkably improved

however the average yield strength is slightly decreased

and the fracture morphology is characterized by cleavage fracture with plastic deformation. Further more

the plastic deformation is decreased with the increase of distance from tensile rupture. After ageing treatment

the average tensile strength and average yield strength of the alloy are higher while the elongation is lower than that of solid solution treated ones. And the fracture morphology is characterized by brittle cleavage fracture

which is created in weak bonding interface and in lamellar structure of β-Mg17Al12 precipitates.