Abstract: Multi-wall carbon nano-tubes (MWCNT′s) were purified and functionalized by refluxing in HNO3-H2SO4(1+1) for 6 h. A dispersion of 5 mg of the MWCNT′s in 10 mL of high-purity water was prepared by ultrasonic vibration for 20 min, and 10 μL of the dispersion were added dropwise onto the surface of glassy carbon electrode (GCE). After air-drying, the modified electrode (abbreviated as MWCNT′s/GCE) was obtained and used in cyclic-voltammetric determination of phoxim in vegetables, based on its catalytic action on the electro-reduction of phoxim at the electrode. In a supporting electrolyte of acetate buffer of pH 4, a significant reduction peak was observed at 0.78 V (vs.SCE), and linear relationship between values of reduction peak current and concentration of phoxim was kept in the range of 5.0×10-8 to 1.0×10-6mol·L-1. Two samples of vegetables were analyzed for their phoxim contents by this method. Values of average recovery and RSD′s (n=5) found by standard addition method using these samples as matrixes were 100.3%, 96.5% and 3.7%, 3.2% respectively.