Recent advances in nanomaterials have made iron oxide nanoparticles (IONPs) as an innovative approach for the delivery of genes. However, the effectiveness of IONPs-assisted gene delivery is currently suffering from their poor uniformity, which not only exhibits detrimental effect on the magnetic property, but also leads to the poor reproducibility to maintain the optimal gene delivery. To this end, the present study developed extremely uniform 15nm-sized IONPs with a good monodispersity in water phase. These ultra-uniform IONPs exhibit notable potentials for an efficient gene delivery to human mesenchymal stem cells (hMSCs) with the assistance of magnetic force, which partly owes to their abilities to facilitate the cellular uptake, as well as to induce the rapid DNA release and the following nuclear transport. Besides, no significant detrimental effects of these IONPs are observed either on the proliferation or the multi-lineage differentiation potential of hMSCs. The current study highlights the potential advantages of designing extremely uniform magnetic nanomaterials for an efficient and safe delivery of genes to stem cells.