An approach,including the surface acid oxidation and covalent grafting,was proposed to prepare modified multi-walled carbon nanotubes(MWNTs).Using multi-walled carbon nanotubes as a new type of solid phase extraction column,and a novel method was developed for the simultaneous determination of two phenoxyacid herbicides e.g.MCPA and 2,4-D in agricultural products by using multi-walled carbon nanotubes as solid phase extraction adsorbent coupled with high performance liquid chromatography(HPLC).The experimental conditions,including the surface acid oxidation,covalent grafting,SPE column operations and chromatography conditions,were optimized.Under the optimal conditions,the method provided a wide linear range for 2 phenoxyacid herbicides with correlation coefficients of 0.998 4-0.999 5,and the detection limit was 0.020 mg/L.The method was applied to detect the residues of phenoxyacid herbicides in soybean,rice and tea.The spiked recoveries for the samples at three spiked concentration levels of 0.20,5.0 and 50 mg/L were in the range of 81%-105 % with RSDs of 3.2%-8.3 %.The results indicated that using multi-walled carbon nanotubes as solid phase extraction adsorbent was efficient for the purification of the agricultural products(included soybean,rice and tea ).With the advantages of accuracy and sensitivity,the proposed method could meet the requirements for determination of low pesticide residues in agricultural products,and provide a useful approach for the analysis of trace substance of agricultural products.