Abstract: The allocation between above-ground and under-ground biomass is influenced by many factors such as plant species, plant age and climate conditions. It can influence carbon input from plant to soil and the carbon cycle in terrestrial ecosystems. Because it is difficult to measure the root biomass precisely, there are still some uncertainties in the root-shoot ratio (R/S), which is considered an important parameter in carbon cycle models of terrestrial ecosystems. The seasonal dynamics of R/S were investigated in grazed and ungrazed grasslands of the Loess Plateau in 2007, 2008, and 2009, and the relationships between R/S, precipitation, and air temperature were analyzed to provide useful information for prediction of under-ground biomass and carbon cycle models in terrestrial ecosystems. The seasonal dynamics of R/S in both grasslands showed an anti-parabolic pattern, with the minimum values occurring in September. The mean R/S values in the three years in the grazed grassland (3.13, 3.36 and 3.77 respectively) were significantly higher than those in the un-grazed grassland (2.55, 2.56 and 3.13 respectively) (P<0.05); At a seasonal scale, there were significant negative correlations between R/S with precipitation and mean temperature in the last two months (P<0.05). Multiple regression equations were built to better predict R/S.