In this paper, the interrelation between structural and magnetic properties of ultra-thin Mn films epitaxially grown on (001) bcc Fe is studied. The Mn growth and in-plane structure were controlled by electron diffraction (RHEED). The structures of the Mn films were determined by using X-Ray absorption spectroscopy (SEXAFS). Finally, the magnetic properties were studied by using X-Ray magnetic circular dichroïsm (XMCD). All the experiments were performed under ultra-high vacuum. As shown by XMCD experiments, a magnetic transition is observed at 2 Mn monolayers. The analysis of RHEED and SEXAFS experimental results clearly demonstrates that a structural transition comes with this magnetic transition.