Oxygen transfer is limited in aerobic granules by compact microbial spatial structure.Aerobic granules were supposed to be spherical， and kinetic parameters were calculated through tests.A simplified equation of mass balance for steady-state diffusion of oxygen was set up and verified.Effective diffusivity was 0.45×10-9 m2·s-1， through dissolved oxygen(DO)change of deactivated granular sludge. Oxygen specific consumption rate and oxygen half saturation coefficient were 0.10 g O2·(g MLSS)-1·h-1 and 0.65 mg·L-1 through beaker test under the condition of COD 400 mg·L-1 and NH4-N 100 mg·L-1.The higher the oxygen concentration and concentration gradient， the stronger and the further oxygen could penetrate.When the radius of granular sludge was more than 1.5 mm， oxygen could not penetrate， despite oxygen concentration was 4.0 mg·L-1.When r/R was between 0.49 and 0.65， the concentration gradient became zero.When the radius was very small （less than 0.5 mm)， oxygen could fully penetrate into granules.At the same time， Thiele modulus was very small and effectiveness factor was near to 1， oxygen diffusion limitation could be neglected.Effectiveness factor was larger with the increase of oxygen concentration， which made oxygen limitation less.