In this thesis, the mechanism of landslides in loess slope is studied based on a field test in association with laboratory tests and numerical modeling. The field test was carried out in the Heifangtai Plateau in China. Heifangtai belongs to semiarid monsoonal with scarce precipitation. Therefore, agricultural irrigation is the major source of water infiltration for the loess slopes in Heifangtai. The test site was selected at the crest of a steep loess slope with developed cracks, covered by more than 40m loess layer. Ground investigation, including lithological composition, groundwater level, soil sampling and soil permeability, was conducted. A field monitoring program was then performed by installation of inclinometers, piezometers, moisture probes, tensiometers and crackmeters. The field irrigation test with instrument monitoring was conducted at the test site on October 2009, lasting for 12 days. Based on field observation and monitoring results, some typical failures occurred in the field test were analyzed in details, including development of cracks, formation of sinkholes and failures on slope surface. These failures are representative in loess regions and are potential factors of loess landslides. A conceptual model was proposed to reflect soil failures induced by water infiltration along the crack. To characterize the loess behavior to infiltration, a comprehensive triaxial testing program was conducted on trimmed loess specimens sampled in Heifangtai. The fundamental behavior of loess was investigated following different stress paths under both saturated and unsaturated conditions. Based on triaxial tests, the soil parameters including mechanical properties and hydraulic properties were obtained. The finite difference program FLAC3D was adopted in this study, the fluid model of which was extended from saturated flow to saturated/unsaturated flow. A numerical model was built to study the mechanism of soil failures induced by variation of water level in the crack, using a proposed moving boundary ...