The densities of liquid Fe-Ni alloys were measured accurately by combination of an electromagnetic levitation technique and a static magnetic field. The static magnetic field suppressed the surface oscillation of the levitated sample droplet, which reduced the experimental uncertainty in the density measurement. Densities were determined over a wide temperature range and included a supercooled region. The densities of all Fe-Ni alloys investigated vary linearly with temperature over the range of measurements. The excess volumes were slightly positive over the entire composition range. The results were discussed within a thermodynamic framework using relationship between excess volume and thermodynamic properties such as excess Gibbs energy and enthalpy of mixing. The excess volume is correlated positively with excess Gibbs energy and enthalpy of mixing for various binary alloy systems.