A　simply　fabricated　microfluidic　device　integrated　with　a　fluorescence　detection　system　has　been　developed　for　on-line　determination　of　ammonium　in　aqueous　samples.　A　365-nm　light-emitting　diode　(LED)　as　an　excitation　source　and　a　minor　band　pass　filter　were　mounted　into　a　polydimethylsiloxane　(PDMS)-based　microchip　for　the　purpose　of　miniaturization　of　the　entire　analytical　system.　The　ammonium　sample　reacted　with　o-phthaldialdehyde　(OPA)　on-chip　with　sodium　sulfite　as　reducing　reagent　to　produce　a　fluorescent　isoindole　derivative,　which　can　emit　fluorescence　signal　at　about　425　nm　when　excited　at　365　nm.　Effects　of　pH,　flow　rate　of　solutions,　concentrations　of　OPA-reagent,　phosphate　and　sulfite　salt　were　investigated.　The　calibration　curve　of　ammonium　in　the　range　of　0.018-1.8　mu　g/mL　showed　a　good　linear　relationship　with　R-2　=　0.9985,　and　the　detection　limit　was　(S/N　=　3)　3.6　x　10(-4)　mu　g/mL.　The　relative　standard　deviation　was　2.8%　(n　=　11)　by　calculating　at　0.18　mu　g/mL　ammonium　for　repeated　detection.　The　system　was　applied　to　determine　the　ammonium　concentration　in　rain　and　river　waters,　even　extent　to　other　analytes　fluorescence　detection　by　the　presented　device.