A novel nitrogen oxide(NO) sensor fabricated by modifying the only planar surface of a carbon fiber microdisk electrode(CFMDE) with single-walled carbon nanotubes(SWNTs) and Nafion membrane was reported for the first time.The modification of SWNTs dramatically improves the sensitivity of CFMDEs and the detection limit of NO was 4.3 nmol/L.The sensor was successfully applied to the measurement of NO released from single isolated human umbilical vein endothelial cells.