The　relativity　between　biodegradability　and　nonyl　branches　structure　for　the　isomers　of　nonylphenol　(NP)　is　studied　by　the　sequencing　batch　activated　sludge　process　for　NP　accumulating　behavior　analyses　in　water　environment.　The　results　show　that　the　residual　content　of　three　NP　isomers　is　markedly　higher　than　that　of　other　isomers　in　the　effluent　and　activated　sludge.　The　three　kinds　of　isomers　have　relatively　low　biodegradabihty.　The　biodegradability　of　NP　isomers　depends　on　the　chain　length,　the　size　and　number　of　branches　in　nonyl.　Because　of　difference　in　the　branches　of　nonyl,　the　biodegradation　of　NP　through　activated　sludge　process　is　associated　with　selectivity　for　different　kinds　of　isomers.　The　NP　isomers　whose　structure　is　of　longer　branch　chain　or　more　branches　in　nonyl　are　harder　to　biodegrade,　so　they　may　accumulate　in　the　environment.