Abstract: The phenomenon of strain localization widely exists in both laboratory or field tests on over-consolidated clay or sand. The deformation of soils when failure occurs, often localizes in a narrow zone, and the strength of the failure plane cannot be reached at the same time. The overconsolidation ratio, mean pressure, friction angle, temperature and strain rate are the main factors to increase the strength of soils. However, some factors may bring negative influence on soil stablity while increasing the strength of soils. Thus, based on the UH model (unified hardening model), the theoretical solution is used to analyze the phenomenon of bifurcation under plain strain. The impacts of the over-consolidation ratio, mean pressure, critical state ratio, temperature and stain rate on bifurcation strain are systematically studied and further discussed. Same conclusions are drawn in the numerical simulations of stain localization by using the finite differential method under the same condition of plain strain.