Two high Tc superconductor YBa2Cu3O7-δ thin films, the zero resistance temperature of which were 82K and 71K 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 irreversibi-lity line was B*-(1-T*/Tc)3/2 deduced from our data.