A facile solution-based thermal decomposition strategy, using very cheap polyisobutylene succimide (PIBSI) and paraffin oil as a surfactant and solvent, respectively, has been developed for the controllable synthesis of magnetic MnFe₂O₄ and CoFe₂O₄ nanocrystals (NCs) with high dispersibility, uniform shape, and high yield. By fine-tuning the reaction temperature and growth time, the morphology and size of MnFe₂O₄ and CoFe₂O₄ NCs can be simply regulated. It is found that the surfactant PIBSI plays a key role in the final shape of the products due to its long chain with non-polar groups, which can markedly hinder the aggregation of the NCs and thus greatly improve the stability and dispersibility of the products. The results reveal that MnFe₂O₄ and CoFe₂O₄ NCs have good biocompatibility and obvious T₂ contrast enhancement effects have been achieved with the increase of iron concentration. MnFe₂O₄ and CoFe₂O₄ NCs show high longitudinal relaxivity of 165.6 and 65.143 mM⁻¹ S⁻¹ in aqueous solutions due to the positive signal enhancement ability of Fe³⁺ ions, indicating the highly potential to be used as effective T₂ contrast agents for magnetic resonance imaging (MRI).