In this study, dynamic behaviors of cracks under dynamic biaxial stress were investigated. We conducted static and dynamic loading fracture experiments on a magnesium alloy (AZ31B-O) under equitable and inequitable biaxial stress by using a hydraulic high-speed biaxial experimental machine. We processed specimens as the cross type with a crack. Different kinds of cracks were defined by their crack angles. We analyzed the results using the caustic method. The experiments revealed some important findings; there is a negative relation between the fracture toughness value and the plate thickness of the magnesium alloy sheet specimen; the fracture toughness value decreases when the crack has an angle different from the loading direction; the fracture toughness value decreases significantly under inequitable biaxial stress.