Applicability of the diffusion bonding was examined in a superplastic magnesium alloy, AZ31, on two different grain sizes of 28 and 11 μm. In order to investigate the superplastic behavior, the tensile test was carried out at the strain rates from 10⁻⁴ to 10⁻² s⁻¹ at elevated temperatures. These materials showed a superplastic behavior at 673 K. The diffusion bonding tests were carried out in the superplastic region, which is the pressure range from 2 to 10 MPa and for the times up to 10 h at 673 K in air. The post-bonded mechanical properties were estimated by the compression lap shear test in order to determine the optimal diffusion bonding conditions. The diffusion bonded specimens exhibited more than 0.8 of parent material strength at several conditions for both materials. The bonding time on fine grained AZ31 could achieve much faster than that on coarse grained AZ31. Using this result, the comparison was carried out experimental results and previous theoretical diffusion bonding models. Many researchers constructed the theoretical models based on the void growth mechanism, diffusional controlled process, to predict its optimal bonding time and pressure. However, the previous modes were not agreement with experimental result. It was resulted from the previous models include only diffusional controlled process. Therefore, in this study, we developed new theoretical diffusion bonding model both diffusional and plastic controlled processes. From the comparison, this model was good agreement with experimental. Using the theoretical diffusion bonding model and experimental results, the prediction map for high quality diffusion bonding of the superplastic magnesium alloys was suggested.