In the paper, concentrations of 18 persistent organochlorinated pesticides in 12 water samples from Guanting Reservoir/Yongdinghe River Watershed, Beijing, were analyzed by using gas chromatography and electron capture detector (GC-ECD). Pentachloro-nitrobenzene (PCNB) was used as internal standard and 2,4,5,6-tetrachloro-m-xylene (TMX) and PCB-209 were used as surrogate. The method recoveries were ranged from 85% to 110%,with a standard d eviation less than 10%. Persistent organic pollutants, such as HCHs, DDTs, and heptachlor were detected in the waters. Among 13 sampling locations, the total concentrations of 13 pesticides under investigation ranged from 4.2-96.9 ng/L. Among the 13 pesticides, concentration of HCHs and DDTs were 0.09-53.5ng/L and nd-46.8 ng/L, respectively. In comparison with concentrations of HCHs in other rivers in China, concentration of HCHs in this watershed were lower than those found in the Zhujiang River (Pearl River), Songhuajiang River, Haihe River and Baiyangdian Catchments and similar to those found in the Yangtze River and middle reaches of the Liaohe River. Concentrations of DDTs in this w atershed were higher than those found in the Zhujiang River and Haihe River and silimar to those found in the Yangtze River and Jiulongjiang River. The origins of organochlorinated pesticides in the watershed were investigated according to the geographical distribution and the isomers characteristics for HCHs and metabolites of DDTs. The ratios of a-HCH to g-HCH were ranged from 0.29-1.38 and indicated the new pollution sources in the watershed, for example the continual application of lindan. The origin of DDTs should come from industrial sources along the Yanghe River. For example, factories manufacturing organochlorinated pesticides in recent years could discharge wastewater-containing DDTs into the river. Our results showed that there were historical residues and recent discharges of organochlorinated pesticides into the watershed.