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The seasonal variations of soil rill erodibility and their potential affecting factors were investigated by carrying out the experiment of water flumes with adjustable bed gradient and using the soil erosion process WEPP model, which was conducted in two tracts of rehabilitated grasslands in “Grain for Green” history during the grass growing season. One (Tract A) had been planted with Leymus secalimus for three years and the other (Tract B) with Medicago sativa L. for 17 years. Results showed that the soil rill erodibility of Tract A of grassland was increased at first and then declined and again increased during the whole growing season （p＞0.05), while the soil rill erodibility of Tract B of grassland was declined at first and then became stable and again declined significantly during the whole growing season （p＜0.05). The soil rill erodibilities of the two grasslands were varied seasonally, which were mainly affected by soil consolidation, formation of water-stable aggregates and root growth. With the increase of soil cohesion, bulk density, content of waterstable aggregates and grass root density, the soil rill erodibility in either soil was declined exponentially. Seasonal variations of soil rill erodibilities of two grasslands could well be simulated with soil bulk density and root density (R≥0.86，NSE≥0.85). The mean soil rill erodibility of Leymus secalimus and Medicago sativa L. during the whole growing season was much less than that of the control tract, which suggested that the practice of “Grain for Green” in hilly regions of the Loess Plateau of China was beneficial to soil and water conservation. The findings of this study may do duty for data support and theoretic basis in elaborating hydrodynamic mechanisms of the soil rill erodibility processes, evaluating effect of grass roots in fields of “Grain for Green” on soil and water conservation, and rationalizing arrangement of soil and water conservation practices and measures in hilly regions of the Loess Plateau of China.