A series of L1(0) Fe63Pt37 films with controlled thickness (t(FM)) were deposited on MgO(100) substrates for microstructure and magnetization reversal mechanism study. X-ray diffraction measurements show that face-centered tetragonal (200) peak also exists in addition to face-centered tetragonal (002) one, and becomes weak for thick films. High resolution electron microscopy study reveals the existence of periodic misfit dislocations at the FePt/MgO interface and other types of defects such as twins and antiphase boundary inside the film. Out-of-plane initial magnetization shows a slow increase responding to the external magnetic field and then follows a steep increase. The out-of-plane coercivity H-C at room temperature decreases with increasing t(FM) and increases when the angle theta H between the external magnetic field and the film normal direction increases. H-C at theta H = 0 changes as a linear function of temperature for individual samples and the slope decreases with increasing t(FM). In addition, magnetic viscosity measurements show that the fluctuation field at room temperature decreases with increasing t(FM). These phenomena indicate that the magnetization reversal in the L1(0) FePt films should be realized by the motion of weakly pinned domain wall and thus governed by the thermal activation model. The magnetization reversal thermal activation volume and corresponding energy increase with increasing t(FM), as a result of the interactions between domain walls and structural defects can be attributed to the sample microstructural characteristic evolution with t(FM). (C) 2012 Elsevier B.V. All rights reserved.