Abstract: Abstract: Structure characteristic is a typical feature of the unsaturated undisturbed loess, which is an important key factor to affect its strength, deformation properties, and son on, and to distinguish the loess from other special soils. To understand the structure of loess, the key is to find the right way to study the structure. Computerized tomography (CT) technology can detect nondestructively the internal meso-structure in real time, so it is used to be a powerful means for loess structure research. However, it is few reports about the influence of structure on yielding features and the structural evolution characteristics of unsaturated loess during isotropic loading process. In this paper, we took the multifunctional geotechnical triaxial apparatus and the CT technology to research loess structure, and the hydrostatic triaxial compressure tests were carried out by using of 4 undisturbed loess samples whose control matric suction was a constant. When the value of volumetric deformation and water displacement of loess samples reached the expected criteria during loading process, we scanned nondestructively the 4 loess samples with the help of CT technology and got some CT scanning data and images. The structural evolution law of the undisturbed loess during isotropic loading procedure was described by means of the relationship between the macro-scopic mechanical index and the meso-scopic scanning data. In these studies, we found that the structure of undisturbed loess could resist the external load to a certain extent, but the influence of loess structure on yielding characteristic was closely related to matric suction. The CT number increased slowly before sample yielding, however, after sample yielding, the structural effect was reduced obviously and the CT number had a linear growth trend, which indicated that the loess sample entered into the plastic hardening stage. When the suction increased, the structure of undisturbed loess had little effect on the mechanical deformation. When the suction decreased, the water film lubrication between soil particles was obvious, so the effect of the structure fighting against the external load was enhanced obviously. After the net mean stress was applied, the voids and pores of loess samples would decrease, but they would not be closed completely, which was related to the formation of new structures that resisted external loads. According to the CT scanning data, the structural parameters and the structural damage variable were defined during isotropic loading tests, respectively, and then the evolution characteristics of the meso-structure of the undisturbed loess were clarified. In addition, we described accurately the relationships between the structural damage variables and the volumetric strain, and the change of the water phase of loess samples, respectively, and established the damage evolution equation of undisturbed loess structure during isotropic loading procedure. This new damage equation had the characteristics of one parameter and simple form, and could well reflect the influence of the change of solid and liquid phase on the structural damage. The validity of new equation was verified preliminarily by comparison of computation results and test data, which indicated very good agreement between each other. The research results in this paper will provide the reference for establishing the structural constitutive model of undisturbed loess and give a gateway to reasonably analyze loess structure.