Hierarchical and flower-like nanostructures have attained great attention over the past decades due to their unique and intriguing properties. Considering the advantages of their particular structure and properties, we prepared g-C₃N₄/MnO₂ nanocomposite by microwave heating technique without using any catalysts, templates and organic reagents. The longitudinal and transverse relaxation time of as-synthesized nanocomposites were measured by using 0.55 T MRI instrument. Manganese is the primary origin for MRI and MR signals; therefore, different concentrations of the composite were dissolved in water. Relaxivities were measured by the inverse of relaxation times and plotted against the different concentrations of manganese. MRI results showed that the strength of the imaging signal is directly proportional to the Mn concentration, demonstrating that as-synthesized flower-like nanostructures have excellent T₁-weighted MRI performance. This study indicates that the nanocomposite of g-C₃N₄/MnO₂ could be a potential candidate as a bifunctional medical system that combines diagnostic imaging and targeted therapy.