The gravimetric and voltametric methods for determination of non-stoichiometry O/U ratio in uranium dioxide used as nuclear fuel are evaluate in this work. The oxidation of uranium oxide is very complex due to many phase changes and the gravimetric and voltametric methods do not detect these phase changes. To evaluate both methods is requiring to do Rietveld methods by X-ray diffraction data to identify the uranium oxides phase changes. The results show that voltametric and gravimetric methods are precise but not accurate techniques for determination of non stoichiometry uranium dioxide samples.