Biological community structures have long been used to assess the impact of human activity on running waters.Compared to macroinvertebrates and fish community,periphyton community was infrequently used as indicator of stream conditions because of the large number of species and the need for taxonomic expertise.However,periphyton is valuable indicator of stream health because they are relative simple to collect and response rapidly and predictably to changes in stream chemistry and habitat quality.In the present work,we investigated the community structure of periphyton and characteristics of environmental factors among 69 sample sites in Taizi River Basin of Liaoning Province,Northeast China.Using the periphyton assemblages,we assessed stream conditions of Taizi River Basin based on Qualitative Habitat Evaluation Index(QHEI),Diatom Bioassessment Index(DBI) and Periphyton Index of Biological Integrity(P-IBI). The results indicated that the dominant species of periphyton in Taizi River Basin were Melosira varians,Diatoma vulgare,Cymbella tumida,C.hustedtii,Navicula cryptocephala,Gomphonema olivaceum,Achnanthes minutissima,Synedra acus,S.ulna,S.ulna oxyrhynchus,Nitzschia palea,Cocconeis placentula,Scenedesmus quadricauda,Oscillatoria tenuis and Phormidium tenus.There was great difference in the periphyton community structure in Taizi River Basin.Based on principal component analysis,conductivity,suspended solid,velocity,total dissolved solid,hardness,alkalinity,ammonia nitrogen,nitric acid nitrogen and total nitrogen were the main environmental factors in Taizi River Basin.Canonical correspondence analysis showed that conductivity,total dissolved solids and total nitrogen significantly influenced the community structure of periphyton in Taizi River Basin. The metrics of DBI were species richness,diversity index,tolerance index,sensitive species and similarity of community,and the metrics of QHEI were epifaunal substrate,embeddedness,velocity,sediment deposition,channel flow status,channel alteration,frequency of riffles,bank stability,vegetative protection and riparian vegetative zone width.The merits of P-IBI were filtered with box-plots analysis and correlation analysis,and the ultimate index merits of P-IBI were total genera of green algae,relative species richness of sensitive diatoms,relative richness of diatom taxons,relative richness of blue-green algae taxons,percent of Nitzschia,percent of Oscillatoria,percent of motile diatoms,percent of diatoms with handle,Chlorophyll a content of periphyton and cell density of periphyton. The results of stream condition assessment based on DBI,P-IBI and QHEI were in consensus in Taizi River Basin,although assessment results of some tributary were not in compatible.The results of stream condition assessment showed that upstream area of Taizi River Basin were in excellent ecological integrity,while,the downstream area of Taizi River Basin were in severe impairment.The results of correlation analysis showed that there were significant correlations among the scores of DBI,P-IBI and QHEI in sample sites of Taizi River Basin,and moreover,significant correlations were also detected between scores of DBI,P-IBI,QHEI and conductivity,total dissolved solid and total nitrogen.In the paper,we also discussed the relationship between periphyton community structure and environmental factors,and moreover,we evaluated the merit and deficiency of DBI,P-IBI and QHEI in the assessment of stream conditions of Taizi River Basin.