One approach to solve the so-called coercivity problem on Nd-Fe-B permanent magnet is to investigate the magnetization reversal process. The magnetization reversal is a transition event between two energy local minima separated by an energy barrier. The energy barrier Eb is a function of magnetic field H and is generally given as the form of Eb = E0(1 - H/H0)n, where E0 is the barrier height at zero magnetic field, H0 the intrinsic coercivity without thermal fluctuation, n the constant. These parameters strongly depend on the magnetization reversal process, but the experimental determination of n is somewhat difficult. Recently, we have proposed the novel experimental method to determine the value of n based on the magnetic viscosity measurements . The parameters of E0, H0 and n also depend on the field direction with respect to the magnetic easy axis. Thus using a highly oriented sample is indispensable for this analysis. A newly developed hot-deformed Nd-Fe-B magnet with Nd-Cu eutectic diffusion under expansion constraint is a good sample for this analysis, because it exhibits very high coercivity of about 20 kOe and excellent c-axis orientation . In this study, energy barrier of the hot-deformed Nd-Fe-B magnets with and without Nd-Cu diffusion are analyzed and the magnetization reversal process and the temperature dependent coercivity are discussed.