Abstract: As one of the poorest regions in China, the lLoess Plateau has the severe erosion across large areas, which has limited agricultural development beyond subsistence farming. The traditionally cereal-dominant agricultural system driven by extensive tillage and cultivation on slopes has contributed significantly to erosion on the plateau. Alfalfa (Medicago sativa) was introduced in China and has since been cultivated for over 2 000 years now. It is one of the important perennial legumes cultivated on the Loess Plateau which increases livestock production, reduces soil erosion in the annual crop rotation system and improves soil fertility through alfalfa-based pasture crop rotation on slopes and annual cropping in upland plains. In a field experiment conducted in Dingxi City, Gansu Province (a typical semiarid area of the Western Loess Plateau), different growth periods of alfalfa were investigated in relation to soil physical properties. The study also discussed the optimum growth years of alfalfa in the Loess Plateau in the cCentral Gansu Province. Soil samples were taken from the 050 cm soil layers in alfalfa field plots with different growth years (3 years, 10 years and 12 years) and in potato fields. Measurements of soil bulk density, soil aggregate stability, saturated hydraulic conductivity, total organic carbon (TOC) and readily oxidized organic carbon (ROOC) were then recorded. The results showed that soil bulk density was significantly lower and total porosity significantly higher in potato fields than in alfalfa fields for topsoil, but not significantly different for deep soil layers. The content of > 0.25 mm water-stable macro-aggregates, mean weight diameter (MWD) and geometric mean diameter (GMD) of soil aggregates in alfalfa fields increased with alfalfa growth years, and were significantly higher than those in cropland fields for the 030 cm soil depth. Then the percentage aggregate destruction (PAD) of soils in alfalfa fields was significant lower than that in potato fields, which decreased with increasing alfalfa growth years. TOC and ROOC improved growth conditions in alfalfa fields for a certain period of growth. The proportion of ROOC onto TOC wasere 44%–57% and 52%–68% in alfalfa fields and in potato fields, respectively. This indicated that alfalfa fields improved soil organic carbon (SOC) content and as well decreased percent ROOC of to TOC. SOC was significantly higher in alfalfa fields than in potato fields, where it initially increased and then decreased with increasing alfalfa growth period. Therefore alfalfa cultivation on the plateau was significantly beneficial under rainfed cultivationrainfed farming system, including the enhancement of soil structure, changes in SOC fraction, and soil permeability. It was, therefore concluded that the optimum growth period of alfalfa in the semi-arid areas of the Loess Plateau was 10 years. It was recommended to develop alfalfa-cereal rotation system after 10 years of alfalfa growth in order to increase crop production, reduce soil erosion and enhance the sustainability of the agricultural system.