The invention concerns a biocompatible oily ferrofluid comprising iron-oxide based magnetic nanoparticles and an oil phase comprising at least one fatty acid ester, characterized in that said magnetic nanoparticles are surface functionalized by molecules of one or more phospholipids, and in particular a biocompatible oily ferrofluid comprising iron-oxide based magnetic nanoparticles and an oil phase comprising at least one fatty acid ester, said iron-oxide based magnetic nanoparticles forming a colloidal dispersion in said oil phase from a temperature belonging to the range from 20 to 80° C., characterized in that said magnetic nanoparticles are surface functionalized by molecules of one or more phospholipids which do not completely cover the surface of the iron-oxide based magnetic nanoparticles, which in particular ensure a coverage rate of the surface of the iron-oxide based magnetic nanoparticles such that the fatty acid ester(s) present in the oil phase have access to the surface of the iron-oxide based magnetic nanoparticles. The invention also concerns the process for preparing such a biocompatible oily ferrofluid and its use as a contrast agent for magnetic resonance imaging or in the context of a cancer treatment by hyperthermia. Finally, the invention concerns a nanoemulsion comprising such a biocompatible oily ferrofluid.