The effect of thixoforming process on morphologies of silicon particles that affect fracture mode of A356 alloy was investigated.Microstructure and fracture surfaces of thixoformed samples were investigated by image analyzing technique and scanning electron microscopy.A new combination parameter,called silicon density ratio (SDR) index,was introduced.SDR index approximates the collective effects of morphological characteristics of silicon particles on microstructure transparency of alloy in crossing the dislocation.It is suggested that samples with lower SDR index have superior mechanical properties,especially elongation,and consequently intergranular fracture mode.On the contrary,samples with higher SDR index have inferior mechanical properties and fracture path tends to propagate along the cell boundaries leading to transgranular fracture.