Abstract: Vegetation restoration can promote the formation of soil aggregates. The stability of soil structure is related to the water-stable aggregate content. The higher the content of water-stable aggregates, the more is the improvement in the soil erosion-resistance. Different plants have different effects on the water-stable aggregate component of soils. To evaluate the stability of soil structure under different grasslands, we studied different artificial grasslands and natural grasslands, including Medicago sativa, Agropyron cristatum, Caragana korshinskii and Stipa capillata in Lanzhou city, Gansu province. A soil aggregate analyzer was used to measure the size distribution of water-stable aggregates. The aggregate size differences and correlation of particle size distribution and mean weight diameter (MWD) of water-stable aggregate of the four different grasslands were analysed for the 0-40 cm soil depth. The results showed that the most abundant particle size of water-stable aggregates was >2 mm or <0.25 mm. For the 0-20 cm depth the proportion of water-stable aggregates of >0.25 mm particle size in S.capillata grassland was significantly higher than other grasslands (P<0.05). For the 20-40 cm depth, the proportion of water-stable aggregates of >0.25 mm particle size was highest in M. sativa grassland. Hence, S.capillata grassland had the most improved soil structure for the 0-20 cm depth, while the M. sativa grassland had a better effect on soil structure at greater depth in the soil profiles. The MWD value for soil samples from M. sativa grassland decreased 31% from 1.04 mm in the 0-10 cm depth to 0.72 mm in 30-40 cm soil depth. These results indicate that in M. sativa grassland, soil structure improvement does not greatly differ between soil depths. There were extremely significant correlations between the MWD value, the proportion of water-stable aggregate in >0.25 mm particle size category and soil organic matter content. Generally, the MWD value and soil bulk density were significantly and negatively correlated. The results demonstrate that increase in the proportion of soil water-stable macro-aggregate and soil organic matter are effective ways to promote soil structure improvement, and enhance soil structure stability and resistance to erosion.