Based on the first-principles calculation, we have investigated the ion diffusion in the NaxLi2-xMnSiO4 (x = 2, 1, 0) compounds with a structure of space group Pn. The ion diffusion in the Na2MnSiO4 and Li2MnSiO4 structures, are studied and compared with each other. In Na2MnSiO4, the Na+ ions migrate through 3D diffusion channels, similar to the Li+ migration in Li2MnSiO4, but with relatively lower activation barriers. The enhanced Na+ conductivity should be attributed to the wider ionic diffusion channels in Na2MnSiO4 than in Li2MnSiO4. In the NaLiMnSiO4 structure, both Na and Li ions are demonstrated to be mobile. Thus, the ionic conduction in NaLiMnSiO4 is thought to be governed by a mixed Li+/Na+ diffusion mechanism. We found the mixed Li+/Na+ diffusion in NaLiMnSiO4 can be also fast, even faster than the Li+ diffusion in Li2MnSiO4.