Abstract: Crack initiation and propagation in ZK60 and ZK60(0.9Y) alloy sheets during dynamic tensile process were investigated by using in situ tensile scanning electron microscope. The relationship between alloy mierostructure and rupture behavior was discussed. Experimental results show that cracks in rolling samples propagate by tearing way. There are cleavage fracture and quasicleavage fracture in the fracture zone. The loading of crack initiation in ZK60(0.9Y) alloy is larger than that of ZK60 alloy. The second phase breaks in the tensile process and the main crack propagates along the second phase. The secondary crack in matrix mostly germinates around the second phase.