We report an in situ study of Fe3Pt-Fe2O3 core-shell nanoparticle growth using liquid cell transmission electron microscopy. By controlling the Fe-to-Pt ratio in the precursor solution, we achieved the growth of nanoparticles with the formation of an iron-platinum alloy core followed by an iron oxide shell in the electron beam-induced reactions. There was no substantial change in the growth kinetics of the iron oxide shell after the Fe-Pt alloy core stopped growing. The core growth was arrested by depletion of the Pt precursor. Heteroepitaxy of Fe3Pt [101] (core)||α-Fe2O3 [111] (shell) was observed in most of the nanoparticles, while a polycrystalline iron oxide shell is developed eventually for strain relaxation. Our studies suggest that Pt atoms catalyze the reduction of Fe ions to form the Fe3Pt alloy core, and when Pt is depleted, a direct precipitation of iron oxide results in the core-shell nanostructure formation.