It is challenging to investigate the magnetic anisotropy of Fe/Si(001) film in the case a limited magnetic field strength, when both coherent rotation and domain wall displacement coexist in the magnetization reversal process. Owing to the domain wall displacement, the magnetization reversal switching field is far lower than the magnetic anisotropy field, and, consequently, only the magnetization reversal process near easy axis can be treated as coherent rotation. Here, we record the slope of the magnetic torque curve of an iron film grown on a Si(001) substrate measured near the easy axis by anisotropic magnetoresistance (AMR) to separate the coherent rotation of magnetization reversal process from domain wall displacement. Furthermore, the magnitudes of various magnetic anisotropy constants were derived from the magnetic torque curves. Our work suggests that the AMR at low fields can clearly separate the detailed contributions of various magnetic anisotropies when domain wall displacement existed in Fe(001) ultrathin film. We also report on the hysteresis behavior of such films as measured by magneto-optic Kerr effect.