Research in AZ61 magnesium alloy has founded superplastic behavior with a grain boundary sliding (GBS) mechanism showing a change in the dominant diffusion process, from grain boundary diffusion (D(GB)) to lattice diffusion (D(L)). The material was tensile tested at two different stresses (10MPa and 16MPa). For each stress, different strain rates and temperature were fixed and tensile test until different strain and measured the volume of cavities by density differentiation. From the relation cavity volume-strain was calculated the cavity growth rate parameter (eta). It is observed that the cavitation growth faster at D(GB) process than for D(L). Also, it was observed a reduction in the volume of cavities with the increment of stress for both diffusion process with a change of the maximum cavitation volume from D(L) to D(GB) at high strains.