Superplastic-like behavior was found in a commercial large-grained Mg-3Al-1Zn alloys (AZ31), at high temperature and high strain rate regime. The creep behavior of this material was studied from RT to 550 degreesC and the strain rates of 0.001 s(-1) similar to1 s(-1), and the maximum elongation to failure of 170% was obtained at 0.01 s(-1) and at 500 degreesC (or 0.84 T-m), which is much higher than the equivalent temperature of most known superplastic materials. Optical microscopy, transmission electron microscopy (TEM) and scanning electron microscopy with electron backscattering diffraction technique (SEM-EBSD) were used to investigate the microstructure and deformation mechanisms involved. It is evident that high temperature and high rate dislocation creep played a critical role at the early stage deformation in breaking down the initial large grains without fracture, and combined GBS and dislocation creep define the steady-state grain size and control the quasi-superplastic behavior.