Abstract: At present, the mechanical design of non-nuclear steel support in AP1000 plant follows the Specification for Structural Steel Buildings (AISC 335 and AISC N690), in which the stability analysis adopts the Effective Length Method （ELM）. While the latest version of Specification for Structural Steel Buildings (ASIC 360-2010) prefers the Direct Method （DM）, and ELM as its alternative. In addition, compared to AISC 335-1989, AISC 360-2010 explicitly requires the consideration of the nonlinear second order effects and initial defects in the stability analysis of the structure support. This paper described the requirements of stability analysis in AISC 360-2010, and the characteristics of DM and ELM. Then using a steel support frame as an example in GTStrudl, the research and comparison of the two methods is conducted. For simple structures, the two methods are applicable; for complex structures, Direct Method is simpler and more efficient.