This study evaluated the effects of low NDF cassava residues and high NDF gramineous forages,com straw stalk,elephant grass and sugarcane tail silage as substrates on in vitro fermentation and microbial community.In vitro fermentation was carried out in a 100 ml glass syringe.Four replicates of every substrate were set up.Each replicate was anaerobically dispensed with 10 ml of rumen fluid,20 ml of McDougalls buffer and 200 mg of dried substrate,and incubated in a water bath at 39℃.The fermentation process was ceased at 24 h of incubation,gas production was recorded and samples of fermented inoculum were collected.Concentrations of volatile fatty acids were measured using gas chromatography,microbial population was quantified by using real time PCR,and microbial community was analyzed by using high throughput sequencing technology.The results showed that,in vitro incubation not only decreased population of bacteria,fungi,methanogen and some cellulolytic bacteria(P<0.05),but also increased diversity of bacteria,reversed the Firmicutes to Bacteroidetes ratio,and greatly decreased the abundance of Prevotella,M.gottschalkii and Entodinium,as compared with those in fresh rumen fluid.The gas production,acetate/propionate ratio,abundance of Succiniclasticum,Entodinium and Diploplastron were the highest,while total volatile fatty acids concentration,fungal and cellulolytic bacterial populations,and abundance of Methanomassiliicoccales and Ostracodinium were the lowest on incubation of cassava residues as compared with other three high NDF gramineous substrates(P<0.05).In vitro incubation with different substrates induced changes in both microbial population and community,especially cassava residues decreased fungal and cellulolytic bacterial population precipitously,and increased Succiniclasticum and Diploplastron abundance as compared with the other three high NDF gramineous substrates;indicated NDF content is important to determine microbial population and community in vitro.Three gramineous substrates appeared similar fermentation patterns and influence on microbial population and community,while sugarcane tail silage had higher abundance of Methanomassiliicoccales and Diplodinium than the other two.This study concluded that in vitro incubation decreased population and abundance of ruminal cellulolytic microbes which can lead to decrease of fiber digestibility in vitro.NDF content of substrate is the most crucial factor to determine microbial population and community in vitro,while other nutrients such as pectin may also influence.