© 2022 The Society for Biotechnology, JapanSelf-assembling ferritin protein cages have been used as a template for magnetic iron oxide nanoparticle synthesis within its 8 nm cavity to be explored as a potential magnetic resonance imaging contrast agent. Here in, magnetic nanocores with various iron content were successfully synthesized using recombinant human H-chain ferritin (HFn) by a controlled mineralization reaction. r1 and r2 relaxivities of the synthesized magnetoferritin nanoparticles were measured and the effect of iron loading factor on the r1 and r2 relaxivity was investigated by using a quite large range of 10 different iron loadings per protein cage (500–5000) at 90 MHz and 300 MHz. The sample with the highest iron loading of 5329 Fe/cage has r2 value of 165.2 mM−1 s−1 and r1 value of 1.98 mM−1 s−1 at 300 MHz. This high r2 value together with a very low protein and iron concentrations (0.03–0.2 mg/mL and 0.15 mM, respectively) renders magnetoferritin very effective T2 contrast agents. However, r1 values were found to be smaller than literature values suggesting that magnetoferritin may not serve as T1 contrast agent in MRI. Moreover, magnetoferritin showed an increase in r2 relaxivity with the iron loadings while r1 values have not been affected by the number of Fe atoms loaded as much as r2 values. This result also sheds light on understanding the formation mechanism of iron oxide core and its contribution on relaxation in MRI.