Boundary effects in magnetization reversal induced by dipole-dipole interaction in micro-square ferromagnetic films are studied by means of spin dynamics simulation.Fixing strength of dipole-dipole interaction,coercive force is calculated as functions of strength of uniaxial anisotropy and magnetization angle θ (the angle between applied field and easy axis).Typical hysteresis loops and spin configurations are given.It shows that shape of hysteresis loop and magnitude of coercive force are determined by a competition between boundary pinning effect and uniaxial anisotropic field,which leads to different coercivity mechanism under different θ.An effective five-magnetic-domain model is proposed to describe spin configurations of square ferromagnetic thin films.The model shows conveniently geometric and physical properties of square ferromagnetic thin films with uniaxial anisotropy as well as its magnetization reversal process.