The thermo-physical properties of magnetic fluid (solvent, water; nano-particulates, magnetite) were elucidated in order to examine its temperature dependency in the presence and absence of a magnetic field. Heat transfer by natural convection was also investigated around a horizontal thin wire, which was heated uniformly in the presence and absence of a magnetic field in order to estimate the heat transfer characteristics. Various thermal properties of the magnetic fluid and their temperature dependency were clarified in the present research. It was also found that the heat transfer coefficient coincides well with the conventional heat-transfer correlation of natural convection heat transfer around a horizontal heated thin wire by applying the values of thermo-physical properties obtained in the present measurement.