The self-forming dynamic membrane coupled bioreactor (SFDMBR), which uses coarse pore-sized material to separate solids and liquids in a bioreactor, has some advantages compared with membrane bioreactor (MBR) using micro-/ultra-filtration membranes such as low module cost and high flux. In this study, we investigate the microbial activity change of a self-forming dynamic membrane (DM) during its bio-fouling process by a microelectrode for O2. At a high flux of 40 L/m2h, the dissolved oxygen was determined to be depleted at the depth of 1.5-2.0 mm in the self-forming DM. Based on the dissolved oxygen concentration profiles in the self-forming DM, a reliable and simple model and computational procedure were developed to estimate the biokinetic parameters in the self-forming DM. Sensitivity analysis of the model revealed that the dissolved oxygen profiles are sufficiently sensitive to the maximum specific rates of oxygen uptake (q down curvemax,20), which were computed to be within the range of 3.8-11.1 mg O2/gSS h. q down curvemax,20 decreased sharply in the first 5 days with the development of the bio-fouling process in the surface cake layer of the self-forming DM and then reached a relatively steady state afterwards. The Monod half-saturation coefficient for oxygen (Ko) was computed to be in the range of 0.16-0.75 mgO2/L. In summary, the results gave new experimental evidence on the change of microbial activity in the self-forming DM during its bio-fouling process.