We investigate the energy barrier E that determines the thermal stability factor Δ, of CoFeB/MgO magnetic tunnel junctions (MTJs) with a perpendicular easy axis by retention time measurement as a function of temperature Tand junction diameter D. E decreases with increasing T for all the studied MTJs. The temperature at which any particular value of E is obtained tends to decrease with decreasing D, indicating that the magnetization reversal in the MTJs is described by reversal models other than the nucleation model. We find that the temperature dependence of E correlates with that of spontaneous magnetization through the power law scaling relationship, which enables us to determine Δ at temperatures different from those used in the retention time measurement.