The structure and magnetism of epitaxially grown Fe films on Cu(100) at 300 K have been investigated in situ using high energy electron diffraction, low energy electron diffraction, scanning tunneling microscopy, and the magneto-optical Kerr effect. Based on a careful thickness calibration, the three well-known regions with distinct magnetic properties are determined accurately for Fe films on Cu(100). We claim that the traditional region II should be further divided into two parts II(1) and II(2), where the former remains region II in the traditional sense but the latter is a transition region between regions II and III. A theoretically predicted temperature driven Martensitic phase transition from face-centered-cubic (fcc) Fe to body-centered-cubic Fe is indeed realized in the transition region II(2).