Abstract: Plots in the Leymus chinensis steppe at the Inner Mongolia Grassland Ecosystem Research Station (IMGERS) were fenced in 1979, 1999, and 2004. The effect of enclosure duration on plant functional groups, species diversity and species composition was studied by testing biomass, coverage, density and height. There were differences in species composition, community characteristics and species diversity with increasing enclosure duration until finally a steady state was reached. The cumulative dominance values of perennial grasses, shrub and sub-shrub from the steppes fenced in 1999 and 1979 were significantly (P＜0.05) higher than those fenced in 2004. Similar results were also observed for forbs. The Shannon-Weaver index, richness index and evenness index of the steppes fenced in 1999 and 1979, were remarkably higher than those of 2004. There was no significant difference between the 1999 and 1979 results. The total biomass, and biomass of forbs, perennial grass, shrubs and sub-shrubs of the 1999 and 1979 plots all showed an increasing trend. The total biomass, shrub and sub-shrub biomass of 1999 and 1979 were significantly higher than those of 2004, while the shrub and sub-shrub biomass of 1979 was considerably higher than that of 1999. There was no significant difference in total biomass between the 1999 and 1979 biomasses. There were significant (P＜0.05) differences between the 1999, 1979 and 2004 biomasses and the order was 1979＞1999＞2004. It is concluded that these results will lead to a decrease in grassland use value and grassland degradation for invasion by shrubs and sub-shrubs.