Abstract: By using temperature-and-density dependent atomic models, a method for uncertainty of electron EOS of plasmas is described via 35 functional exchange-correlation potentials. A fast method for roughly evaluating uncertainty is designed, and checked in Thomas-Fermi model. Taking gold as an example, calculations of uncertainties of total inner energy, electron pressure and ionization degree, with their 'quasi true value', are carried out in the range of temperatures from 1.0 to 10 000.0 eV with three densities 1.0, 10.0, 100.0 g·cm^-3, respectively. The results can be applied in engineering design.