In　this　study,　sequencing　batch　reactor　(SBR)　and　aerobic　upflow　sludge　bed　reactor　(AeUSB)　were　used　to　investigate　the　effects　of　sludge　granulation　on　establishing　partial　nitrification　(PN)　process　and　explore　the　potential　mechanism.　The　particle　size　was　positively　related　to　the　NO2--N　accumulating　performance,　satisfactory　nitrite　accumulation　rate　(NAR)　of　96.4%　and　77.3%　were　obtained　with　the　appearance　of　microgranules　in　SBR　and　AeUSB　after　day　105　and　83,　respectively.　To　our　knowledge,　it　is　the　first　report　describing　the　importance　of　extracellular　polymeric　substances　(EPS)　content　and　components,　such　as　aromatic　protein-like　substances,　by-product-like　substances　and　special　protein　secondary　structures　(alpha-helices　and　3-turn　helices)　for　sludge　granulation　and　even　PN　process.　Quantitative　microbial　analysis　suggested　that　ammonium　oxidizing　bacteria　(AOB)　predominated　in　nitrifying-bacterium　in　micro-granules　system,　guaranteeing　a　highly　efficient　PN　process.