The　simultaneous　nitrification　and　denitrification　in　step-feeding　biological　nitrogen　removal　process　were　investigated　under　different　influent　substrate　concentrations　and　aeration　flow　rates.　Biological　occurrence　of　simultaneous　nitrification　and　denitrification　was　verified　in　the　aspect　of　nitrogen　mass　balance　and　alkalinity.　The　experimental　results　also　showed　that　there　was　a　distinct　linear　relationship　between　simultaneous　nitrification　and　denitrification　and　DO　concentration　under　the　conditions　of　low　and　high　aeration　flow　rate.　In　each　experimental　run　the　floc　sizes　of　activated　sludge　were　also　measured　and　the　results　showed　that　simultaneous　nitrification　and　denitrification　could　occur　with　very　small　size　of　floc.