We investigate the magnetic configurations during the magnetic reversal of permalloy nanodisks varying in diameter and thickness, by means of OOMMF simulation software ,which employs the finite difference (FD) method to solve Landau-Lifshitz-Gilbert (LLG) equation. Depending on the disk size , many (meta)stable magnetic patterns such as vortex, and various buckle states are observed. The results Suggest that the disk size is the decisive factor in determining which states is more favorable than others, and that the disk diameter determines whether some in-plane buckling can be sustainable.We also conclude that there is a distinct behaviour switch between the vortex state and the single-domain state,dependending on the diameter of the nanomagnet.