To optimize solid electrolytes for the Na-ion batteries, it is highly critical to understand the effect of the grain boundary on the overall Na-ion transport. Therefore, a large-scale MD simulation was carried out on the three-twist and two-tilt GB structures and the crystalline and amorphous NASICON solid electrolyte structures. The Na-ion diffusion pathways depict the interruption in Na-ion movements at various GBs. As a result, at GBs, tracer diffusion coefficients are lower, and activation energies are higher than in grain and crystalline samples. The Na-ion pathways at the grain boundary demonstrate that Na-ion movement is similar to an amorphous sample. The radial distribution function analysis shows that the GB and amorphous local structures are not identical, and these GBs have a local structure comparable to grain and crystalline samples. The GB area density was increased compared to the grain, while the distance between the Na-ions was reduced. This decreased distance between Na-ions does not reflect Na-ion accumulation at the GBs, as would be the scenario in an amorphous sample.