An on-line improved ultraviolet detection method was developed for the determination of melamine in various samples by using water plug coupled with field-enhanced sample injection and capillary zone electrophoresis technique（FESI-CZE）.The optimal conditions were as follows:water plug containing 60% water and 40% acetonitrile,water injection time:200 s,running buffer:120 mmol/L NaH2PO4（pH 2.2）- 10% methanol,sample matrix:0.10 mmol/L NaH2PO4（pH 2.2）- 20% acetonitrile,sample injection voltage:20 kV,sample injection time:80 s,separation voltage:20 kV.Under the optimum conditions,the calibration curve was linear in the range of 10-1 000 μg/L.Compared with that of the traditional CZE method,the sensitivity of FESI-CZE method was improved about 800 times and the LODs(S/N=3) was decreased from 2.0 mg/L to 2.5 μg/L.This method was used to detect the melamine in various samples with recoveries of 98%-106% and RSDs（n=4）no more than 5.1%.This method is sensitive,reliable,economic and pretreatment free for complex samples,and it could overcome the shortcoming of low sensibility with ultraviolet detector.