Nitrification stability and biofilm robustness were examined by comparing a fibrous support membrane-aerated biofilm reactor (FS-MABR), where a woven fibrous support was surrounded on a silicone tube, with an MABR. The overall mass transfer coefficient of oxygen for the FS-MABR, assuming no boundary layer between the fibrous material and bulk liquid, was 5.85 m/d at an air pressure of 27 kPa, which was comparable to that value of the MABR (5.54 m/d). The amount of biomass on the fibrous support with a silicone tube was 2.48 times larger than on the bare silicone. The biomass loss after a high liquid flow rate condition was 49% and 75% in the FS-MABR and MABR, exhibiting robust biofilms grown on the fibrous support. The FS-MABR provided more stable nitrification performance than the MABR irrespective of a high liquid flow rate. Both reactors have deteriorated ammonium (NH4+-N) removal without a high liquid flow rate condition to eliminate excessive biomass, indicating that regular maintenance is essential to eliminate excessive biofilm from a MABR for nitrification, which potentially acts as a NH4+ diffusion barrier.