Abstract: In this paper, model of oxygen potential and control theory of atom ratio of oxygen and metal (O/M ratio) for FBR MOX fuel were discussed. Blackburn model and point defect model are usually used two types of oxygen potential model of nuclear fuel, whose precision is affected by ion reaction equilibrium constant, thermodynamic data and experimental measured data. If a designed O/M ratio of 1.97 is required for (U_0.75Pu_0.25)O_2-x fuel sintered at 1 750 ℃ in 0.1 MPa Ar-5%H₂ mixture gases, oxygen partial pressure will be limited to 1.07×10^-5 Pa or oxygen potential will be limited to -386.15 kJ/mol when calculated by Blackburn model; oxygen partial pressure will be limited to 0.70×10^-5 Pa and oxygen potential will be limited to -393.22 kJ/mol when calculated by point defect model. If a designed O/M ratio of 1.95, 1.96, 1.97, 1.98, 1.99 and 1.995 is required respectively, water content in gas will be limited to 370.4, 739.8, 1 633.7, 4 403.6, 17 855.4 and 43 064.8 ppm respectively, or gas dew point will be limited to -30.10, -23.27, -14.98, -3.77, 13.83 and 26.16 ℃ respectively.