Two high T_c_ superconductor YBa_2Cu_3O_7_ thin films, the zero resistance temperature of which were 82K and 7lK respectively, were selected for measurement about resistance broadening. The difference in thermal activation energy of the two samples was analyzed with Arrhenius law auggested by Palstra. We found that the activation energy decreased depending upon increasing magnetic field. The irreversibility line was B * -(l -T*/T_c_)**3/2 deduced from our data.