We report an in situ study of the time evolution of magnetic anisotropy constants of an uncapped 4 nm [∼27 monolayers (ML)] Fe film epitaxially grown on a GaAs (110) substrate at room temperature under ultra-high vacuum (UHV) conditions. The structural and chemical properties are monitored by low energy electron diffraction and Auger spectroscopy with a sensitivity of 0.01 ML. The in situ UHV ferromagnetic resonance (FMR) study over a period of 6 days in <10⁻⁹ Pa reveals that there is a slow magneto-morphological transition of the Fe film surface at room temperature. The resonance field measured in situ in the [110] direction initially changes at a rate of 0.3 mT/h within 30 h after deposition and later at 0.1 mT/h over 80 h. We determine the time-dependent changes in the in-plane and out-of-plane anisotropy constants and find a sign change in the uniaxial in-plane anisotropy in the first 24 h due to morphological changes at the surface. The in situ FMR measurements and the Auger analysis allow us to exclude changes in the magnetization and anisotropy due to the contamination and oxidation of the Fe film.