A novel and easy method was developed for the separation of 5 nucleosides by capillary electrophoresis in co-electroosmotic mode using a polymeric ionic liquid(PIL)，poly(1-vinyl-3-ethylimidazolium bromide) as background electrolyte modifier.The polymeric ionic liquid，as a cationic polymer，was electrostatically attached to the negatively charged surface of the fused-silica capillary，generating a reversed electroosmotic flow.Effects of polymeric ionic liquid concentration and pH on the electroosmotic flow were studied.Effects of several factors such as background electrolytes(BGES) concentration and pH value on separation were also investigated.Under the optimized conditions，good separation of 5 nucleosides was achieved with high column efficiency and excellent reproducibility in 3.1 min.Meanwhile，the method was compared with capillary electrophoresis background electrolyte system without or with ionic liquid(IL or PIL)as additive.The results showed that the separation time of 5 nucleosides was reduced sharply and the separation efficiency was improved after adding PIL.The highest column efficiency was up to 950 000 plates/m.RSDs(n=5) of the migration time for the analytes were all no more than 0.38%.The method is simple，efficient and excellently reproducible and has a good application prospect.