A eco-friend analytical method by using a microdialysis extraction device coupled sub-critical water ion-pair liquid chromatography has been investigated to analyze phenoxy acidic herbicides (2,4-D,MCPP,and 2,4,5-T) in aqueous samples.Phenoxy acidic herbicides in aqueous samples (at pH 2.0) were extracted with 1-octanol through an on-line microdialysis sampling device,and then analyzed by the self-assembled sub-critical water HPLC-UV system eluting with methylamine in water as ion-pair reagent.Conditions influencing extraction efficiency and sub-critical water ion-pair liquid chromatography were thoroughly optimized.Experimental results indicated that extraction of aqueous sample solutions (30 ml) at pH 2.0,by static micro-dialysis extraction for 30 min.,and then flowed into sample-loop of injector with the flow rate of 1 μl.min-1 for 17 min,resulted in the good extraction efficiency.The sub-critical water ion-pair liquid chromatography was achieved by using PRP-1 polymer column at 140(o)C and eluting with water containing 10 mM.L-1 methylamine (pH 11) at the flow rate of 0.8 ml.min-1.Detection limits were 2,4,and 6 μg.L-1 for 2,4-D,MCPP,and 2,4,5-T,respectively.The real samples were collected from river in an agriculture area and analyzed with recovery between 70-85%.The present method provides a simple,and eco-friendly procedure for sampling and determining phenoxy acidic herbicides in aqueous samples