By understanding microstructure and properties of thixo-molded AZ91D fabricated at different processing parameters,the fracture behavior was investigated.The results show that ambient microstructure of typical thixo-molded AZ91D alloy is mainly composed of un-melted α-Mg and solidified phase.The un-melted solid phase can be divided into four types,such as spherical solid particles,irregular solid particle,solid particle with liquid pool inside and solid particle with liquid alloy inside.Solidified phase includes primary α-Mg phase and eutectic phase,in which primary phase exhibits regular structure with fine grain,controlling by under-cooling rate of liquid alloy.Porosity rate is the main factor influencing the mechanical properties of the alloy with similar microstructure,while effects of microstructure on the mechanical properties of the alloy with similar porosity rate is dominant.The samples with a lower solid fraction and a finer primary solid phase exhibits acceptable properties.Crack in the sample during tensile testing is propagated along interface of un-melted solid phase and liquid phase or along primary solid phase and eutectic phase.With a higher solid fraction,crack is mainly propagated along interface of un-melted solid phase and liquid phase,resulting in the deterioration of mechanical properties,while with a lower solid fraction,crack is mainly propagated along interface of primary solid phase and eutectic phase,leading to the improvement of mechanical properties as a result of bridging effects though β phase.