The　presence　of　recalcitrant　substances　in　wastewater　poses　challenges　for　water　treatment,　as　together　with　other　readily　biodegradable　substrates,　they　negatively　affect　the　treatment　of　actual　wastewater.　In　this　study,　the　differential　effects　of　a　typical　representative　of　recalcitrant　substances　(i.e.,　oleic　acid)　on　the　performance　of　activated　sludge　system　and　microbial　community　structure　at　different　concentrations　(0　%,　50　%,　and　100　%)　were　examined.　According　to　the　results,　when　the　influent　contained　no　oleic　acid　(0　%),　the　best　sludge　settling　performance　was　achieved　with　a　sludge　volume　index　(SVI)　of　98　mL/g;　this　was　superior　to　systems　with　oleic　acid　influents　of　50　%　(SVI　=　290　mL/g)　and　100　%　(SVI　=　146　mL/g).　The　addition　of　oleic　acid　significantly　increased　the　content　of　extracellular　polymeric　substances,　especially　extracellular　protein.　This　enhanced　microbial　activity　but　also　led　to　poor　sludge　settling　performance,　particularly　in　the　50　%　oleic　acid　system,　where　the　protein/polysaccharide　ratio　increased.　Microbial　community　analysis　revealed　that　100　%　oleic　acid　promoted　the　growth　of　denitrifying　bacteria,　such　as　Hydrogenophaga　and　Rhodoferax,　while　inhibiting　Candidatus_Accumulibacter　and　Dechloromonas.　In　the　50　%　oleic　acid　system,　the　filamentous　bacterium　Thiothrix　spp.　dominated,　causing　sludge　bulking.　In　the　0　%　oleic　acid　system,　the　growth　of　Zoogloea　facilitated　floc　formation　and　enhanced　settling.　These　findings　reveal　that　oleic　acid　alters　microbial　metabolism　by　shifting　microbial　carbon　source　utilization　and　increasing　EPS　production,　which　in　turn　alters　microbial　community　dynamics　and　negatively　impacts　sludge　settling.　This　study　provides　insights　for　optimizing　the　treatment　of　oleic　acidcontaining　wastewater.