In molecular imaging studies via magnetic resonance imaging, in vivo cell tracking is an important issue for the observation of cell therapy or disease behavior. High resolution imaging and longitudinal study are necessary to track the cell movement. Since the field inhomogeneity extends over several voxels, we have performed the numerical analysis using the sub-voxel method dividing a voxel of MR image into several elements and the information about the field inhomogeneity distribution around the micro-beads. We imbedded ferrite-composite micro-beads with the size of $20-150{mu}m$ in the subject substituted for cells to induce local field distortion. In the phantom imaging with the isotropic voxel size of $200{mu}m^3$, we could confirm the feasibility of sub-voxel tracking in a 3.0 T MRI.