Abstarct: A simple and sensitive flow injection-electrochemical detector procedure was developed for the on-line determination of trace levels of Cr(VI) in the presence of Cr (III). The performance of glassy carbon electrode (GCE) coated with poly(methylene blue) –multiwalled carbon nanotube (MWCNT) composite was investigated in the determination of Cr(VI) using online amperometry. The presence of MWCNT and poly methylene blue improves the sensitivity and reproducibility of the electrode. The effect of several parameters including pH, volume and type of buffer, type of electrolyte, number of polymerization cycles, concentration of methylene blue and MWCNTs, ionic streghnth, applied potential, flow rate and loop length on the electrochemical determination of the analytical were studied and optimized. Under the optimum conditions, calibration curve were linear over the concentration range of 0.2-35 µM for Cr(VI). Also the detection limit (DL) of 0.07 µM was obtained for Cr(VI). The effects of interfering ions were studied and it was found that the sensor has least of interference from most of the common ions due to selective interaction of the modifier functionalities with Cr(VI). The response of the electrode remained constant for at least 7 days of successive opration. Finally the proposed method was successfully applied for the determination of Cr(VI) in different water samples.