Diffusion of iron atoms in a thin FePt film of L1(0) structure was investigated in the low temperature region where iron diffusivities are between 10(-22) m(2) s(-1) and 10(-24) m(2) s(-1). A new method using nuclear resonant scattering of synchrotron radiation in grazing incidence geometry was used, providing higher accuracy and sensitivity than the conventional tracer methods. Isotopical FePt multilayer samples Pt(20 A)/[(FePt)-Fe-57(20 A)/FePt(30 A)](10)/MgO(001) produced by molecular beam epitaxy were annealed at four temperatures between 773 and 873 K for times between 60 and 120 min. The nuclear reflectivity was measured at room temperature after each annealing step and the decrease of the nuclear superstructure Bragg-peak intensities was observed. From the intensity loss, the diffusion coefficient and the activation energy for iron self-diffusion in the FePt thin film along the c axis were determined as D-0=(3.45 +/- 0.44)x10(-13) m(2) s(-1) and Q=(1.65 +/- 0.29) eV. The value for the activation energy is the same as found by residual resistivity measurements in the same system.