Based　on　lab　scale　experiments,　factors　including　temperature,　biofilm　and　assimilable　organic　carbon　influencing　biostability　of　water　after　ultrafiltration　were　investigated.　All　results　showed　that　a　small　amount　of　bacteria　existed　in　water　after　ultrafiltration.　Within　the　water　temperature　5-22　degrees　C,　the　higher　the　temperature　was,　the　higher　of　TTC　dehydrogenation　activity　was　for　these　bacteria　and　the　higher　heterotrophic　plate　count　bacteria　was　found.　In　water　distribution　pipes,　bacteria　could　left　biofilm　which　attached　to　pipes　wall　and　suspended　in　bulk　water　in　favorable　conditions.　Thus,　biostability　of　drinking　water　decreased.　In　the　absence　of　disinfectants,　the　biostability　of　water　treated　by　ultrafiltration　with　the　assimilable　organic　carbon　concentration　less　than　13　mu　g/l　was　ideal　and　acceptable.