[en] The fundamental magnetism of the Fe film of 4 ML thick epitaxially grown on GaAs(100) was studied using ex situ ferromagnetic resonance (FMR) at room temperature. In parallel geometry the resonance fields were found to be strikingly different along [0 bar 11] (2170 Oe) and [011] (3360 Oe) direction, exhibiting a large uniaxial anisotropy with the easy and hard directions along [0bar 11] and [011], respectively. For perpendicular geometry the data of FMR show an abnormally low resonance field of 5050 Oe. It implies a pronounced reduction of effective magnetization. This can be explained by a combination of the shape anisotropy of the nanoclusters with their large perpendicular anisotropy. The g factor of these nanoclusters is even smaller than the bulk value. The linewidth is also obviously anisotropic, 220 Oe in easy direction, [0bar 11] and smaller by 50% in hard direction, [011]. [copyright] 2001 American Institute of Physics