Abstract: The field investigations and an experiment were conducted in the Juma river in the suburbs of Beijing to study the effect of habitat fragmentation on river ecology,using benthic macro-invertebrates as indicator species.Three experimental plots were isolated from a relatively undisturbed stream habitat with sheet iron.The benthic assemblages and water parameters were measured by sampling periodically.The results indicate that the abundance,taxa richness and biodiversity of inverte-brates significantly decrease in the experimental plots owing to the habitat isolation.The smaller the experimental habitat plot,the more significantly these biotic indices decrease.The contents of the dissolved oxygen in the studied plots present the inconsistent variations.The comparison of the benthic communities shows that the relative abundances of Ephemeroptera and Diptera reduces significantly in the isolated plots,and that of the Odonata and Lamellibranchia increase significantly.It is also found that the benthic communities need some time to stabilize after isolation,and then present apparent variation over time.There is a relatively high degree of taxa turnover between isolated plots and the non-isolated reach,which can be attributed to the flight and dispersal of many aquatic insects in their adult stage.However,the benthic communities in isolated plots are not nested subsets in the natural non-isolated stream.This paper also gives some suggestions of the river restoration and the preservation of river ecological integrity based on the study and the present status of the rivers in China.