This paper discusses in detail the deformation textures, glide system, petrofabrics and olivine dislocation microstructures of mantle peridotites at Yushigou in the North Qilian Mountains, northwestern China. The peridotites have undergone high-pressure, high-temperature and low-strain rate plastic flow deformation. According to the dynamic recrystallized-grain size of olivine and the average spacing between the dislocation walls as well as the chemical composition of enstatite, the authors calculated the rheological parameters of the ancient upper mantle in the study area as follows: temperatures 1025-1093℃; pressures 3043-4278 MPa; depths 95-132 km; deviatoric stress 28-32 MPa; strain rates 0.2×10-14-2.13×10-14s-1 and equivalent viscosities 0.45×1020-4.65×1020 Pa · s. These parameters suggest that the position where plastic flow took place was correspondent to the low-velocity zone beneath the oceanic lithosphere and that oceanization characterized by middle-velocity (1-3 cm/a) sea-floor spreadi