Thin Fe films, 1 nm to 90 nm, have been grown on GaAs (100) substrates in ultra high vacuum, base pressure = 2×l0−8 Pa. The growth of the films has been followed with Auger electron spectroscopy and a recently developed in-situ UHV M/H hysteresis loop tracer. The Auger signal provides an indication as to when the Fe covers the GaAs substrate or when it clusters into three dimensional islands. The magnetic hysteresis loop tracer provides the coercivity and the saturation magnetic moment, from which the saturation magnetization or average film thickness can be obtained. The coercivity of thin films is sensitive to structure, strain, and to the grain size, in analogy to its sensitivity to particle size for small particles. The data show that the coercivity, Hc, as a function of film thickness, is very sensitive to the annealing procedures and to the resulting morphology, continuous versus local clustering of the iron film. Under proper conditions a clear maximum in Hc versus film thickness is observed for these films near 5 nm thickness, in analogy to three dimensional small particle curves.